Ionic Radii

It is convenient to think of ions as spheres having a definite radius. When two ions are brought close together, a force of repulsion between them sets in abruptly when they are a certain distance apart and resists any closer approach. The distance between their centers is then taken as the sum of the radii of the two ions. In this way, the ions are treated as spheres in contact, and by various methods their radii can be measured. The following list of ionic radii is due largely to the work of V. M. Goldschmidt, J. A. Wasastjerna, and others; it is arranged so that the columns correspond to the groups of the Periodic Table, and ionic charge is indicated at the top of each column.

Notice particularly the large size of the oxygen ion (1.32) in comparison with relatively small positive ions (cations) such as Si, AI, Mg, and Fe. It is the spacing of the oxygens, which are closely packed together in a silicate crystal that largely controls the scale of the structure; the smaller cations are situated in the interstices between the oxygens. Other negative ions (anions), e.g. F-, Cl- -, S- -, are similarly large.

The hydrogen ion is exceptional in being extremely small; when bound to an atom of oxygen it becomes embedded, as it were, in the oxygen, and the resulting (OH)-ion has about the same radius as that of oxygen. We can think of the H-ion as a centre of positive charge without dimensions.

When one or more electrons are removed from a group of atoms bound together by homopolar bonds, a charged radicle is formed. Thus the carbonate ion, C0₃- -, has one carbon atom to which are attached three evenly spaced oxygen atoms, and it carries a double negative charge. The sulphate ion, 504- -, has four oxygens arranged at the corners of a tetrahedron around a central atom of sulphur. The shapes of these ions affect the type of crystal structure of their compounds. Thus in calcite the Ca-ions are situated at the corners of a rhombohedron and the CO₃-ions, represented by triangles in the figure, are arranged with their centres midway along the rhombohedral edges and lie in horizontal planes. Physical and optical properties parallel and perpendicular to these planes differ greatly.

Co-ordination

In ionic crystals each positive ion (cation) is surrounded by a number of negative ions, at a distance fixed by the sum of their radii. The number of negative ions around anyone cation is called the co-ordination number, and is determined by the ratio of the radii of the two kinds of ion. Thus in sodium chloride, the relative sizes of the sodium and chlorine ions are such that every sodium is surrounded by six chlorines arranged at the corners of a regular octahedron, a grouping known as 6-fold co-ordination. The metals aluminium, iron, magnesium and titanium, among others, are also found in 6-fold co-ordination.In silicate minerals, silicon always has four oxygen atoms arranged around it at the corners of a tetrahedron; the space left between the four oxygens packed together thus is just sufficient to accommodate the small silicon ion. Other cations which are found in 4-fold co-ordination are beryllium and zinc. An important feature of many silicate structures is that aluminium, already mentioned as having 6-fold coordination, can also play the role of silicon (because its ionic radius is only a little larger) and thus occur in 4-fold co-ordination. Aluminium is, because of its size, a borderline case and can occur in either 4- or 6- co-ordination.

Groups with co-ordination numbers from 7 to 12 are formed by large cations such as Na, Ca, K, Sr, Ba, and Zr, but these larger groups tend to be less regular than the smaller groups. The tetrahedral and octahedral groups (4 and 6- co-ordination) are very regular in their form, and in building crystals they are put together so that neighboring groups share corners, edges, or faces, and are thus linked together so as to build up the pattern.

X-rays are somewhat like light waves but have a much shorter wave-length (see p. 149), this being comparable to the distances between atoms in a crystalline solid. When a beam of X-rays falls on a crystal, it is scattered or diffracted by the layers of atoms within the crystal, in the same way that light waves are diffracted by an optical grating. In making an analysis of a crystal structure, the diffracted X-rays are allowed to fall on a photographic plate, and the resulting photograph shows a series of spots or lines which form a more or less symmetrical pattern. From measurements made on the photograph, the arrangement of the atoms in the crystal can be deduced and also the distances between them. Distances are expressed in Angstrom units; one Angstrom (Ã…)=10-8 em. The several methods of taking X-ray photographs of crystals or of powdered minerals cannot be discussed here, but the principle is broadly that outlined above; details can be found in books on the subject. We are more concerned with the results of X-ray analysis, as these have thrown a flood of light on the structure of crystals, and in particular of minerals, and have confirmed the classes of symmetry worked out in the past by crystallographers from a study of external form, as previously. X-ray analysis has been especially helpful in connection with the big group of mineral silicates, and in this field the work of many investigators has given us a considerable, though not yet complete, knowledge of their structures.

The Unit Cell

Every crystal consists of certain atoms or groups of atoms arranged in a three-dimensional pattern, which is repeated throughout the crystal. The smallest complete unit of pattern is called the unit cell, and the whole pattern is formed by stacking unit cells together. To take a simple example, in crystals of sodium chloride (NaCI) the atoms of N a and CI are arranged at the corners of a series of cubes, as shown in Fig. 68. The unit cell of sodium chloride contains four atoms of Na and four of CI, whose arrangement is exactly similar to that in every other unit cell of the substance. It is to be noted that the number of atoms in the unit cell of a particular mineral is not necessarily the same as in its formula, but is usually some simple multiple; for NaCI this multiple is four. Verify by counting the atoms in Fig. 68, allowing for some atoms belonging half to the unit cell shown there and half to adjacent cells. The array of points in space at which the pattern repeats is called the lattice.

Atomic Bonds

There are four main kinds of bond which hold together the atoms in different crystal structures; they are known respectively as the ionic or polar bond, the homopolar or co-valent bond, the metallic bond, and the residual or van de Waals bond. Correspondingly, crystals may be divided into four classes, each characterized mainly by one of the above four types of bond. The van der Waals bond is very weak in character and is present in all crystals. Nearly all the common minerals have ionic bonding.

In this most general case, the unit form cuts the three axes at unequal lengths from the origin, and this fact is often indicated loosely by stating that the crystallographic axes of this type of crystal are of unequal lengths.

In some crystals the unit form cuts two axes at an equal distance and the third at a different distance. In this case, the axes cut at equal distances are both called a and the third, placed vertical, is called c. It is customary to say here that the two axes are equal and the third different.

Again, in other crystals, the unit form cuts all three axes at the same distance, so that all the axes are interchangeable; in this case the axes are all called a, and are loosely said to be equal.

The position in space of the faces of a great number of crystals can be referred to three crystallographic axes, but in one group four axes are used.

The planes in which two of the crystallographic axes lie are called the axial planes.

Crystallographic Notation

Crystallographic notation is a concise method of writing down the relation of any crystal face to the crystallographic axes. The most widely used systems depend upon either parameters or indices Of these systems of notation, the chief are two,-the Parameter System of Weiss, and the Index System of Miller (modified by Bravais).

Parameter System of Weiss

In this system of crystallographic notation, the axes are taken in the order explained above,-that is, a, b , c, for unequal axes, a, a, c, for two axes equal, and a, a, a, for three axes equal. The intercept that the crystal face under discussion makes on the a-axis is then written before a, the intercept on the b-axis before b, and the intercept on the c-axis before c. These intercepts are of course measured in terms of the intercepts made by the unit form on the corresponding crystallographic axes.

The most general expression for a crystal face in the Weiss notation is

na, mb, pc,

where n, m, p are the lengths cut off by. the face on the a, b , c axes as compared with the corresponding lengths cut off by the unit form. It is usual to reduce either n or m to unity.

If a crystal face is parallel to an axis, it can be imagined as cutting that axis at an infinite distance, and accordingly the sign of infinity, 00, is placed as its parameter before the corresponding axial letter . Thus a face cutting the a-axis at a distance 1 unit,-that is at the same distance as the unit form cuts this same axis,-and cutting the b-axis at a distance 2 units or twice the distance cut off by the unit form along the b-axis, and running parallel to the c-axis has the Weiss symbol

a, 2b, c.

A face cutting the a-axis and parallel to the b-axis and c-axis obviously has the symbol

a, b, c.

Crystals in which like faces are equally developed and are equal distances from the centre of the crystal are rare; but for convenience of study and of representation by diagrams, it is necessary to deal with crystals in their simplest and due to some restraint on directions or to a greater supply of material being available in one direction as compared with another, most intelligible form, and that is when they have perfect geometrical symmetry.

Most crystals occur in distorted forms, having like faces not of the same size and not in the same geometrically symmetrical position. In many cases of distorted crystals the crystallographic symmetry has been made out from the fact that like faces have like properties. Etch-marks produced by solvents acting on the crystal faces, the behaviour towards heat and electricity, the hardness, luster, etc., of the faces, have revealed the true symmetry of the distorted crystals. This is illustrated in the quartz crystal shown in Fig. 14, where the etch-marks are similar on like faces.

Distortion in crystals may be growth of the crystal in certain directions or to a greater supply of material being available in one direction as compared with another.

The term habit is used to denote the characteristic shapes of crystals arising from variations in the number, size and shape of the faces; the distorted octahedron shown in Fig. 13 has a tabular habit; in Fig. 15 are shown two habits of apophyllite crystals.

Crystallographic Axes

In solid geometry the position of a plane in space is given by the intercepts (or the lengths cutoff) that the plane makes on three given lines called axes. This method of treatment is employed in crystallography, and the axes are termed the crystallographic axes. Whenever there is present a suitable number of axes of symmetry they are chosen as the crystallographic axes. The crystallographic axes intersect at the origin.

Parameters

The parameters of a crystal face are the ratios of the distances from the origin at which the face cuts the crystallographic axes, - that is, the parameters are the ratios of the intercepts. In Fig. 16, OX, OY, OZ, represent the crystallographic axes, and ABC is a crystal face making intercepts of OA on OX, OB on OY, and OC on OZ. The parameters of the face ABC are given by the ratio of OA, OB, and OC. It is convenient to take the relative intercepts of this face as standard lengths for the purpose of representing the position of any other face, such as DEF. In the case of the face DEF, OD is equal to OA, OE is twice OB, and OF is half OC, and therefore 1/1, 2/1, 1/2 are the parameters of DEF with reference to the standard face ABC.

The form whose face is taken as intersecting the axes at the unit lengths which are to be used for measuring the intercepts made by other forms on the same axes is called the fundamental, parametral or unit form. The selection of a suitable unit form depends on the properties and nature of the crystals. A form well developed, or commonly occurring, or parallel to which there is a good cleavage, is usually selected for this purpose.

The parameters of the unit form can be obtained by measurement, and can be expressed as multiples of one of their number. Take, for example, gypsum. It is found that the most commonly occurring form in gypsum crystals which makes intercepts with all three crystallographic axes does so in the ratio of 0′6899:1 :0,4124. This expression is called the axial ratio, and simply means that the standard or unit form cuts one axis at a distance represented by 0′6899, the second axis at a distance represented by 1, and the third axis at a distance represented by 0′4124. When we use this unit form to measure the intercepts, or to obtain the parameters, of any other form that cuts all three axes we shall do so by taking 0•6899 as our unit of measurement along the first axis, 1 along the second axis, and 0′4124 along the third axis.

Indices

The reciprocals of the parameters are called the indices and are of use for purposes of crystallographic notation.

]]> http://mineralogy.frequentlyasked.info/2007/09/19/crystallographic-and-geometrical-symmetry-2/feed/ http://mineralogy.frequentlyasked.info/2007/09/18/law-of-the-constancy-of-interfacial-angles/ http://mineralogy.frequentlyasked.info/2007/09/18/law-of-the-constancy-of-interfacial-angles/#comments Tue, 18 Sep 2007 18:58:21 +0000 admin http://mineralogy.frequentlyasked.info/2007/09/18/law-of-the-constancy-of-interfacial-angles/ It has already been mentioned that crystals are built up of an orderly arrangement of the atoms or atomic groups of the mineral. Examination of crystals by X-rays has led to the determination of the relative positions of the different atoms in the structure. If the atoms are represented by points, their arrangement in the crystal can be shown by a geometrical pattern or framework which is called the space-lattice or point-system. In this, the atoms are arranged in innumerable parallel rows which intersect in a regular pattern. The rows lie in planes to form what may be called a net-plane. Crystal faces are parallel to these net-planes, and crystal edges to the rows of atoms occurring at the intersections of net-planes.

We have seen that the atomic structure for the crystals of anyone mineral is fixed, so that it follows that the positions of the faces of such crystals are fixed also. This leads to the enunciation of the important law of the Constancy of Interfacial Angles. The corresponding interfacial angles are constant for all crystals of a given mineral, provided, of course, that the crystals have identical chemical compositions and that the measurements are made at the same temperatures.

Zones

Inspection of many crystals shows that their faces are so arranged that the edges formed by the intersections of certain of the faces are parallel with one another. Such a set of faces constitutes a zone, and the line with which the edges are parallel is called the zone-axis. For instance, the common crystals of quartz or rock crystal such as are illustrated in Fig. 121, show six faces meeting in parallel edges, and terminated by a set of six usually triangular faces which do not meet in parallel edges; the first set of six faces forms a zone.

Symmetry

Examination of a crystal either with the eye or a gonimeter shows that there is a certain regularity of position of like faces, edges, etc. This regularity constitutes the symmetry of the crystal. The degree of symmetry varies in different minerals and is employed, as seen later, in the classification of crystals. It is defined with reference to three criteria of symmetry:

• Plane of Symmetry
• Axis of Symmetry
• Centre of Symmetry

Plane of Symmetry

A plane of symmetry divides a crystal into two similar and similarly placed halves. In other- words, such a plane divides the crystal so that one half is the mirror-image of the other. Planes of symmetry can be illustrated by considering a cube. A cube has nine planes which divide it into two halves so that one half is the reflection of the other. The traces of these nine planes are indicated on the faces of the cube in Fig. 9 and the dissected planes are shown in Fig. 10.

The geometrical symmetry of a matchbox or a brick is obviously lower than that of a cube for, as inspection shows, there are only three planes that divide the object into similar and similarly placed halves.

Axis of Symmetry

If a crystal, on being rotated, comes to occupy the same position in space more than once in a complete turn, the axis about which rotation has taken place is called an axis of symmetry. Depending upon the degree of symmetry, a crystal may come to occupy the same position two, three, four or six times in a complete rotation. The terms applied to these different classes of axes are as follow:-

Two times: two-fold, diad, half-turn or digonal axis.

Three times: three-fold, triad, one-third-turn or trigonal axis.

Four times: four-fold, tetrad, quarter-turn or tetragonal axis.

Six times: six-fold, hexad, one-sixth-turn or hexagonal axis.

We can again use the cube and our brick to illustrate axes of symmetry. In the cube, as shown in Fig. ii, there are axes of four-fold, three-fold and two-fold symmetry. Rotation of the cube about the axis of four-fold symmetry shown in the figure causes the cube to take up the same position in space four times during a complete rotation, about the three-fold axis three times, and about the two-fold axis twice. It ‘is clear, moreover, that there are three axes of four-fold symmetry, four of three-fold symmetry and ,six of two-fold symmetry in the cube. This is expressed in the following way:

Axes of Symmetry of the cube - 3^iv, 4ⁱⁱⁱ, 6ⁱⁱ.

In our brick there are only three axes of symmetry and these are of two-fold type; they connect the middle points of the pairs of opposite faces of the brick.

Centre of Symmetry

A crystal has a centre of symmetry when like faces, edges, etc., are arranged in pairs in corresponding positions and on opposite sides of a central point. The cube and our brick obviously have centers of symmetry.

The Symmetry of Gypsum as an Illustration

A crystal of gypsum may be taken to illustrate these definitions of symmetry. The usual form of such a crystal is shown in Fig. 12.

There is one plane which divides the crystal into two similar and similarly placed halves. This plane is the only plane of symmetry for this crystal. At right angles to this plane is an axis of symmetry. Rotation about this axis causes the crystal to take up the same position twice in a complete rotation, and this axis is therefore an axis of two-fold symmetry. Lastly, for every face, edge or corner that occurs in one half of the crystal there is a similar face, edge or corner in a corresponding position in the other half. Therefore the crystal has a centre of symmetry.

Thus the symmetry of this gypsum crystal may be expressed in the following way:

• Planes of symmetry
• Axes of symmetry
• Centre of symmetry

]]> http://mineralogy.frequentlyasked.info/2007/09/18/law-of-the-constancy-of-interfacial-angles/feed/ http://mineralogy.frequentlyasked.info/2007/09/17/elements-of-crystallography/ http://mineralogy.frequentlyasked.info/2007/09/17/elements-of-crystallography/#comments Mon, 17 Sep 2007 21:22:37 +0000 admin http://mineralogy.frequentlyasked.info/2007/09/17/elements-of-crystallography/ In was noticed by the ancient Greeks that a certain mineral, quartz, usually occurred in forms having a characteristic shape, being bounded by flat faces. From the transparency of this mineral and the occurrence in it of included material, it was thought that quartz resulted from the freezing of water under intense cold, and hence the name krustallos-meaning clear ice-was given to the substance. There were, however, numerous other minerals known to the ancients which occurred in forms bounded by flat faces, and so, by a natural extension of the term, krustallos came to signify any mineral showing such forms.

By the researches of Steno, De l’Isle and Hauy, the study of crystals gradually evolved from mere speculation. It is to Hauy that we are indebted for an illuminating theory of the structure of crystals. Hauy argued that crystals were built up of minute bricks of the mineral, different modes of arrangement of the bricks producing different crystal forms. By more recent investigations Hauy’s notion of the arrangement of material bricks has been replaced by that of the arrangement of atomic groups. It is therefore apparent that chemical constitution has an important influence on crystalline form, and, indeed, Von Federov issued a list of some ten thousand substances, the chemical composition of which he was able to tell with certainty from an examination of their crystals.

The study of crystals is called crystallography. Crystals are bodies bounded’ by surfaces, usually flat, arranged on a definite plan which is an expression of the internal arrangements of the atoms. They are formed by the solidification of minerals from the gaseous or liquid states or from solutions,-a process known as crystallization.

From the definition of a crystal just given we see that the internal atomic structure is their fundamental property. Though we could construct a model of a crystal in glass or some other amorphous material, such a model would not be a crystal since it would lack the essential atomic structure. In this book, however, we are chiefly concerned with the determination of minerals, so that for us the external form of crystals demands most attention. In this chapter our crystallography will be almost entirely morphological. The atomic structure of crystals is considered in the next chapter.

Characteristics of Crystals

Faces

Crystals are bounded by a number of surfaces which are usually perfectly flat, but may be curved as in some specimens of siderite and diamond. These surfaces are called faces. Faces are of two kinds, like and unlike. Some crystals are limited by faces that are all alike. For instance, fluor-spar commonly crystallizes in cubes, and any one face of the fluor-spar cube is like all the other faces in its properties. Such faces that have the same properties are called like faces, whilst faces having different properties are unlike faces.

Forms

A crystal made up entirely of like faces is termed a simple form. For example, the cube and the octahedron are each of them simple forms, since all the faces of each have the same properties. The front face shown in the drawing of a cube in Fig. 5 can be replaced by any other of the cube faces without altering the drawing. A crystal which consists of two or more simple forms is called a combination. In Fig. 5, the cube and the octahedron are shown as simple forms and also as a combination such as occurs in crystals of galena.

Some simple forms occur by themselves in crystals as they can enclose space, but others can only occur in combinations, since they have too few faces to enclose space by themselves. Such latter forms are called open.

Edge

An edge is formed by the intersection of any two adjacent faces. The position in space of an edge depends, of course, upon the positions of the faces whose intersection gives rise to it.

Solid Angle

A solid angle is formed by the intersection of three or more faces.

Interfacial Angle

The angle between any two faces of a crystal ‘is termed the interfacial angle. In crystallography, the interfacial angle is the angle between the normals, or perpendiculars, to the two faces. The interfacial angle between the two faces shown in section is A. Interfacial angles are of great importance in crystallography and are recorded in works of reference in the following way, if the angle between the normals to two faces which we will call m and mIII is 630° 48′ it is recorded as mIII = 630° 48′.

Measurement of Interfacial Angle

The interfacial angles of crystals are measured by the goniometer (or angle measurer). Two types of this instrument are used, one termed the contact-goniometer, the other the reflecting goniometer.

The contact-goniometer consists of two straight-edged arms movable on a pivot or screw, and connected by a 3 graduated are, as shown in Fig. 7. These two arms are brought accurately into contact with adjacent faces of the crystal, and the angle between them read off on the graduated arc. In the illustration, the angle actually measured is the internal angle between the two faces, and this must be subtracted from 1800 to give the interfacial angle of the crystallographer.

Reflecting goniometers are rather elaborate instruments used with crystals possessing perfectly smooth or flawless faces. In general, the smaller the crystal, the more suitable for use with the reflecting goniometer will it be.

A common form of .reflecting goniometer consists of a vertical circle, graduated and capable of rotation, and a horizontal arm fixed at right angles to the plane of the circle. A mirror is fixed on the horizontal arm. The crystal is placed at the centre of the graduated circle with an edge parallel to the horizontal arm. The image of a distant signal is observed by reflection from the mirror, and also by reflection from the crystal face. By rotating the graduated circle and with it the crystal, the two images are made to lie in the same straight line. The circle is then rotated until an image is obtained by reflection from the adjacent face. The amount of rotation gives the angle between the normals to the two crystal faces, that is, the interfacial angle, as shown in Fig. 8. Here light reflected from the face AB of the crystal in the ABCD position is seen by the eye. If the crystal is rotated about the edge between AB and AD so that the face AD takes up the new position dA where dA and AB are in the same straight line, then the signal is again seen. The crystal has been rotated through the angle dAD, which is the supplement of the internal angle between the faces B AB and AD, and is therefore the interfacial angle.

]]> http://mineralogy.frequentlyasked.info/2007/09/17/elements-of-crystallography/feed/ http://mineralogy.frequentlyasked.info/2007/09/14/specific-gravity/ http://mineralogy.frequentlyasked.info/2007/09/14/specific-gravity/#comments Fri, 14 Sep 2007 15:54:04 +0000 admin http://mineralogy.frequentlyasked.info/2007/09/14/specific-gravity/ The specific gravity of a body is the ratio of the weight of the body to that of an equal volume of water. This latter weight varies with the temperature, and this variation has to be considered in exact work.

In the general practice of determinative mineralogy, however, this correction can be neglected. In selecting material for the determination of specific gravity it is necessary to obtain as pure a sample as possible and one free from alteration products, inclusions and the like.

The cardinal principle employed in most determinations of specific gravity is that the loss in weight of a body immersed in water is the weight of a volume of water equal to that of the body. If W, is the weight of the body in air, Ww its weight ‘in water, then Wa - Ww is the weight of the water displaced by the body and the specific gravity of this is

Wa

---

Wa - Ww

Methods of Determining Specific Gravity

The following are the chief methods of determining specific gravities in mineralogy, the particular method chosen depending usually upon the size and character of the specimen under examination.

1. With the ordinary chemical balance, for fragments of a solid mineral about as big as a walnut.
2. With Walker’s steelyard, for large specimens.
3. With Jolly’s spring balance, for very small specimens.
4. By measuring the displaced water, for the rapid determinatfon of the approximate specific gravity of a number of specimens of a mineral.
5. With the pycnometer or specific gravity bottle, for friable minerals, small fragments or liquids.
6. With heavy liquids, used mainly for the separation of mineral mixtures into their pure components according to their specific gravities, but also for approximate determinations of specific gravity of mineral grains. For this latter determination, the diffusion column and Westphal Balance may be employed.

1. Determination of Specific Gravity with the Chemical Balance - The mineral is weighed on a good chemical balance. It is then suspended by thread or very fine wire from one arm of the balance and immersed in water contained in a beaker standing on a wooden bridge placed over the scale-pan. Bubbles of air sticking to the mineral are removed by a small brush, and the weight of the mineral immersed in water obtained. The specific gravity of the mineral is given by dividing its weight in air by the difference between its weights in air and water.
2. Walker’s Steelyard - This instrument is useful for determining the specific gravity of large specimens. The essential part of the apparatus is the long graduated beam which is pivoted near one end and counterbalanced by a heavy weight suspended to the short arm. The specimen is suspended and moved along the beam until it counterbalances the constant weight, the level position of the beam being observed by a mark on the upright shown on the right of the figure. The reading (a) is taken. The specimen is then immersed in water and moved along the beam until the constant weight ‘is again balanced and a second reading (b) is obtained. The readings (a) and (b) are inversely proportional to the weights of the body in air and in water respectively. Hence

   

   whence the spec’ific gravity is given by dividing the second reading by the difference between the second and first readings.

3. Jolly’s Spring Balance - This instrument consists of a spring suspended vertically against a graduated scale. To the lower end of the spring are attached two scale-pans, one below the other, the lower scale-pan being always immersed in water. The reading (a) of the bottom of the spring on the scale is obtained. A small fragment of the mineral is placed in the upper pan, and a second reading (b) taken. The specimen is then transferred to the lower pan, and a third reading (c) taken. Then (b - a) is proportional to the weight of the mineral in air, and (b - c) to the loss of weight in water, so that

   

4. Measurement of the Displaced Water - The specific gravity of a large number of pieces of a uniform mineral may be rapidly obtained with a fair amount of accuracy by half filling with water a graduated cylinder of suitable size, and placing therein the previously weighed specimens, and noting the increase of volume. The weight in grammes of the mineral in air, divided by the increase in volume in cubic centimetres, gives the specific gravity of the mineral.
5. The Pycnometer or Specific Gravity Bottle - The pycnometer is used to obtain the specific gravity of liquids or of small fragments of minerals, gems, or porous or friable material. It is a small glass bottle fitted with a stopper through which is a fine opening. When filled up to a certain mark or to the top of the stopper, the bottle contains a known volume of liquid, so that by weighing the bottle empty and then filled with liquid, the specific gravity of the latter can be obtained. If the volume of the bottle is not known, the specific gravity of a liquid may be determined by weighing the bottle empty, then filled with water, and finally filled with the liquid, whence it is clear that the specific gravity of the latter is given by dividing the weight of the liquid by that of the water, since their volumes are the same.In determining the specific gravity of mineral fragments, the mineral is first weighed. The bottle is filled with distilled water. Both the mineral and the filled bottle are placed in the same scale-pan and their combined weight obtained. The mineral is then put into the bottle from which it displaces an equal bulk of water, and the weight again determined. The weight of the water displaced is given by subtracting this last weight from the preceding. The specific gravity is obtained by dividing the weight of the mineral by the weight of the water it displaces.
6. The Use of Heavy Liquids - If a mixture of two minerals of different specific gravities is placed in a liquid whose specific gravity lies between those of the minerals, the heavier mineral sinks in the liquid and the lighter mineral floats and thus a more or less complete separation of the two minerals can be effected. Further, by varying the specific gravity of a liquid a point can be reached when a given mineral placed in the liquid neither floats nor sinks; the specific gravity of the mineral and that of the liquid are then the same and by determination of the latter the specific gravity of the mineral can be obtained. These two principles are the basis of the use of heavy liquids in mineralogy and petrology.

Heavy liquids are used for the purification of mineral material for analysis, for the separation of a rock into its component minerals and especially for the separation of the small amount of minerals of relatively high specific gravity, the heavy residues or accessories, in certain rocks. For all these purposes, the mineral or rock must be disintegrated by crushing, use of acids, etc., until particles composed of single minerals alone are present. Dust is washed off and at various stages the material is sieved. The prepared material is placed in the heavy liquid contained in a separating funnel. The simplest form, and the best, of this apparatus consists of an ordinary filter funnel to which is attached a short length of rubber tubing capable of being closed or opened by a press-clip. The mixture of material and liquid is gently stirred, or agitated by pressing the tubing above the clip. Minerals lighter than the liquid float to the top, and those heavier sink to the bottom and can be drawn off through the tubing. By varying the specific gravity of the liquid, a pure separation can be obtained.

In the determination of the specific gravity of a mineral by heavy liquids various methods are used. In the first, the heavy liquid is diluted until the mineral neither sinks nor rises in the liquid but remains suspended. The specific gravity of the liquid, and therefore of the mineral, is determined by means of the pycnometer (if there is a large amount of the liquid) or by using the Westphal Balance. In this, a sinker is immersed in the liquid and balanced by riders on a graduated arm. The arm is usually so graduated that the specific gravity of the liquid can be read off directly.

For testing the specific gravity of small samples the diffusion column is used. Two perfectly mixable liquids of different specific gravities are placed in a graduated tube without mixture, and allowed to stand for a day or more until regular diffusion of the two liquids has taken place. Thus is formed a column of liquid in which the specific gravity varies regularly from top to bottom. Small fragments of known specific gravity are placed in the liquid and, coming to rest at particular points in the column, serve as indices. A small quantity of the finely powdered sample is introduced, and its several constituents separate into bands with different specific gravities. The specific gravities of these bands can be told by their positions with regard to the indices of known specific gravity.

]]> http://mineralogy.frequentlyasked.info/2007/09/14/specific-gravity/feed/ http://mineralogy.frequentlyasked.info/2007/09/13/mohs-scale-of-hardness/ http://mineralogy.frequentlyasked.info/2007/09/13/mohs-scale-of-hardness/#comments Thu, 13 Sep 2007 16:43:00 +0000 admin http://mineralogy.frequentlyasked.info/2007/09/13/mohs-scale-of-hardness/

Hardness	Standard Mineral
1	Talc
2	Rock-salt, or Gypsum
3	Calcite
4	Fluor-spar
5	Apatite
6	Orthoclase Felspar
7	Quartz
8	Topaz
9	Corundum
10	Diamond

Window-glass may be used in an emergency as a substitute for apatite, and flint for quartz.

The hardness test may also be made by endeavoring to scratch the specimens enumerated ‘in the list with the mineral under examination. If, for example, the mineral scratches orthoclase felspar but does not scratch quartz, it has a hardness between 6 and 7. A greater precision is sometimes attempted by giving the hardness as 6 ¼, 6 ½, 6 ¾, according to whether the mineral in question approaches more nearly to felspar or quartz in hardness.

Hardness may also be tested by means of a penknife or even the finger-nail, the former scratching up to about 6 ½, the latter up to 2 ½. Finger-nails, however, vary in hardness.

Several precautions are to be observed in testing hardness. A definite scratch must be produced in the softer mineral and this is best seen by blowing away (or licking away, if the observer cares to) the powder produced by scratching and then examining the place with a lens. A softer mineral drawn across a harder mineral often produces a whitish stripe which may be mistaken for a scratch in the harder mineral; in the same way an attempt to scratch harder minerals with the knife produces a steel mark on them. Granular specimens may give a kind of scratch by the breaking out of the mineral grains. Finally, it is of course necessary that a fresh surface, that is, one not coated with decomposition products or the like, of the mineral is subjected to the hardness test.

During the hardness trial, the color of the powder produced by the scratch is observed, this giving the streak of the mineral.

Tenacity

Minerals possess certain properties dependent upon their tenacity, of which the following are the most important:

1. Sectility - A mineral is said to be sectile when it can be cut with a knife and the resulting slice breaks up under a hammer. Examples: graphite, steatite, gypsum.
2. Malleability - A mineral is malleable if a slice cut from it flattens out under a. hammer. Examples: native gold, silver and copper.
3. Flexibility - is the property of bending. In some minerals it can be observed by experimenting with their plates or lamina, only. A flexible mineral remains bent after the pressure is removed. Examples: talc, selenite, etc.
4. Elasticity - as the term is usually employed in mineralogy differs from flexibility in the fact that the portion bent springs back to its former position. Mica yields flexible elastic plates, whilst the somewhat similar mineral, chlorite, gives plates that are flexible but not elastic.
5. Brittleness - is a character common to many minerals and is shown by their crumbling or flying to powder instead of yielding a slice. Examples: iron pyrites, apatite and fluor-spar.

Fracture

It is very important to note the characters of the fractures displayed on the broken or chipped surfaces of minerals. It is equally important to distinguish between the smooth flat surfaces resulting from what is called the cleavage of a mineral, and the irregular surfaces characterizing true fracture, these latter being totally independent of cleavage. Whilst the fracture is an important diagnostic character and, further, a recent fracture reveals the true color of certain minerals, it is unwise to break or chip good crystals, as crystalline form is a far more valuable and constant a character by which to determine a mineral than its color and, in many cases, than its fracture.

Fracture is said to be:

1. Conchoidal -The mineral breaks with a curved concave or convex fracture. This often shows concentric and gradually diminishing undulations towards the point of percussion, somewhat are resembling the lines of growth on a shell. Conchoidal fracture is well shown by quartz, flint and natural glasses.
2. Even - The fracture-surface is flattish or nearly fiat, as in chert.
3. Uneven - The fracture-surface is rough by reason of minute elevations and depressions. Most minerals have an uneven fracture. (4) Hackly.-The surface is studded with sharp and jagged elevations, as in cast-iron when broken.
4. Earthy.-As in the fracture of chalk, meerschaum, etc.

Cleavage

The tendency to split along certain definite planes-the cleavage-planes-possessed by many minerals is closely related to crystalline form and the internal structure of the crystal. In each cleavable mineral, the directions of the cleavage-planes are parallel to a certain face or to certain faces of a form in which the mineral may more closely packed together or the mutual electrical charges are greater than in directions at right angles to the cleavage-plane. This plane, therefore, is a plane of least cohesion and hence splitting or cleavage easily occurs along it. It is important, as already stated, to distinguish between fracture and cleavage, as the former is irregular and not connected with the crystalline structure of the mineral. Substances with no crystalline structure, that is, amorphous substances, show no cleavage. Certain rocks, such as slate, which split readily into thin sheets are said to be cleaved, but this property of slaty cleavage, as it is best called, is the result of recrystallization produced by pressure and has no connexion with the cleavage which exists in minerals.

Minerals may cleave in one, two, three or more directions, but one cleavage is generally to be obtained with greater ease than the others. Cleavage is described by stating the crystallographic direction followed by the cleavage-planes and the degree of perfection shown by such planes. With regard to the latter, cleavage is described as perfect or eminent, good, distinct, poor, indistinct, difficult, etc. As examples of minerals with perfect cleavage, we may give fluor-spar, galena, calcite, and mica. Fluor-spar commonly crystallizes in cubes; if such a cube is taken and tapped with a hammer it will be found to cleave along planes truncating the corners of the cube, and if this cleaving is done in a regular wayan octahedron is produced. Fluor-spar is said, therefore, to have a perfect octahedral cleavage, and to give octahedra as its cleavage fragments. Galena, which also crystallizes in cubes, cleaves parallel to the faces of the cube, so that its cleavage is cubic and its cleavage-fragments are cubes. Calcite, no matter what shapes its crystals are, produces rhombohedral cleavage-fragments on being crushed. Mica possesses one perfect cleavage parallel to which exceedingly thin sheets of the mineral may be split off.
Cleavage is a very important property in the recognition of minerals, both in the hand-specimen and, as is shown later, under the microscope.

Gliding-planes and secondary twinning are related to cleavage, and are produced in a mineral by pressure. For example, during the preparation of a thin slice of calcite for examination under the microscope, the pressure of grinding the mineral may cause it to show an excellent cleavage and some secondary twinning. Twinning is discussed in later pages. The secondary twin-planes and the gliding planes are often planes along which the mineral separates fairly readily-such planes are called partings.

Surface Tension Effects

The difference in adhesive power of various liquids to different minerals has formed the basis for numerous processes of ore separation and concentration. The surface tension between various metallic sulphides and oil is greater than that between the gangue minerals quartz, calcite, etc., and the same medium. In the original Elmore Process a paste of sulphide and gangue was mixed with oil and water and agitated; the oil separated into a layer above the water and carried the sulphides with it. Somewhat the same principle underlies the method of extracting diamonds from their matrix, blueground, by causing them, to adhere to grease upon shaking tables. The various Flotation Processes depend on surface tension. In these, bubbles of gas or air attach themselves to say, fine-powdered zinc blende, agitated in oil-mixtures, and float this mineral to the surface, leaving other sulphides and gangue material at the bottom of the liquid. By varying the conditions of flotation clean separations of various ore-minerals can be produced and in this way the working of mixed ores has been made economically possible.

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Gases and liquids

Oxygen, nitrogen and carbon dioxide are examples of natural gases; and water, mercury and petroleum are examples of natural liquids.

Solids

With the exception of mercury and the natural mineral oils, all the minerals with which we have to deal are found in the solid state, and the properties dependent on their state of aggregation are now considered.

Form

Under favorable circumstances minerals assume certain definite geometrical forms called crystals, the recognition of which is a valuable aid in the identification of minerals.

1. Crystallography or the study of crystals is dealt with in the next chapter. The following general descriptive terms are associated with the crystal characters of minerals:
• Crystallized - a term denoting that the mineral occurs as well-developed crystals. Most of the beautiful specimens in museums are of crystallized minerals.
• Crystalline - a term denoting that no definite crystals are developed, but a confused aggregate of imperfectly formed crystal grains that have interfered with one another during their growth.
2. Cryptocrystalline - a general term to denote the possession of mere traces of crystalline structure. Amorphous is used to describe the complete absence of crystalline structure, a condition found in the natural glasses but rare in minerals.
3. Minerals assume various indeterminate forms that are not necessarily dependent on crystal character. These forms are described by the following terms, which have their customary meanings:
• Acicular - in fine needle-like crystals, as in natrolite. Amygdaloidal-almond-shaped, as with the minerals known as zeolites which occupy the almond-shaped steam cavities of lavas.
• Bladed - in forms shaped like a knife-blade or a lath, a form commonly exhibited by many museum specimens of kyanite.
• Botryoidal - eonsisting of spheroidal aggregations, somewhat reo sembling a bunch of grapes, as with chalcedony.
• Capillary - exhibiting a fine hair-like form as in millerite, nickel sulphide, whence the name capillary pyrites or hair pyrites for such varieties of this mineral.
• Columnar - showing a form resembling slender columns, as in horn. blende.
• Concretionary and nodular - terms applied to minerals which are found in detached masses, the forms being spherical, ellipsoidal or irregular, as in the flint nodules found in the Chalk of the south of England.
• Dendritic and arborescent – tree-like or moss-like forms, usually produced by the deposition of the mineral in very narrow planes or crevices, as with the dendrites of manganese oxide.
• Fibrous - consisting of fine thread-like strands, as exhibited by the variety of gypsum called satin-spar, and by asbestos.
• Foliated or, better, foliaceous - consisting of thin and separable lamellae or leaves, as with mica and other micaceous minerals.
• Granular - in grains, either coarse or fine. Evenly granular aggre. gates of minerals, such as in marble, are often termed saccharoidal from their resemblance to lump sugar.
• Lamellar - consisting of separable plates or leaves as with wollastonite.
• Lenticular - with the form of flattened balls or pellets, shown by many concretionary and nodular minerals.
• Mammillated - displaying large mutually interfering spheroidal surfaces, as in malachite.
• Radiating or divergent-showing crystals or fibres arranged around a central point, as in stibnite and in many cases of concretionary forms.
• Reniform - kidney.shaped, the rounded surfaces of the mineral reo sembling those of kidneys and shown in perfection by the variety of hematite called kidney iron-ore,
• Reticulated - in the form of cross• meshes like a net, as with the rutile needles found in some micas.
• Scaly - in small plates as with tridymite.
• Stellate - showing fibres radiating from a centre to produce star-like forms, as with wavellite.
• Tabular - showing broad flat surfaces, as with wollastonite or tabular spar.
• Tuberose - showing very irregular rounded surfaces often giving rise to gnarled, rootlike shapes as in the variety of aragonite called fios-ferri.
• Wiry or filiform - in thin wires often twisted like the strands of a rope, and shown well by native silver and copper.

Pseudomorphism

Pseudomorphism is the assumption by a m’ineral of a form other than that which really belongs to it. Pseudomorphs may be formed in several ways:

1. A pseudomorph by investment or incrustation is produced by the deposition of a coating of one mineral on the crystals of another, for example, quartz on fluor-spar.
2. A pseudomorph by infiltration is formed when the cavity previously occupied by a certain crystal is refilled by the deposition in it of different mineral matter by the infiltration of a solution.
3. A pseudomorph by replacement arises by the slow and gradual substitution of particles of new and different mineral matter for the original particles which are successively removed by water or other solvents. This kind of pseudomorphism differs from the preceding in the circumstance that the new tenant enters before the old tenant has entirely evacuated its quarters.
4. A pseudomorph by alteration is due to gradual chemical change which crystals sometimes undergo, their composition becoming so altered that they are no longer the same minerals, although they still retain the old forms. As an example may be instanced the common alteration of olivine to serpentine.

Pseudomorphs may often be recognised by a want of sharpness in the edges of the crystals, whilst their surfaces usually present a dull and somewhat granular or earthy aspect.

Polymorphism

It has already been mentioned that two minerals of markedly different physical properties, such as colour, hardness, crystal form, specific gravity, etc., may have ‘identical chemical compositions. Such substances are said to be dimorphous and illustrate the general property of polymorphism. The minerals making up a polymorphous series are composed of the same atoms but have them arranged on different plans so that their physical properties differ.

As an example of dimorphism we may take the two forms of calcium carbonate occurring as the minerals calcite and aragonite. These two minerals form crystals of quite different types, their optical properties are different, and aragonite is harder and has a higher specific gravity than calcite. Again, the physically very dissimilar diamond and graphite are dimorphous forms of carbon. (See Fig. 69.) In nature titanium dioxide, Ti02, occurs in three forms or is trimorphous. The mineral anatase has a specific gravity of 3′9, brookite of 4′15, and rutile of 4′25, and their other physical characters are dissimilar, but in chemical composition they are all titanium dioxide. It is probable that the temperature, pressure, concentration, etc., operative at the time of formation of the mineral control what variety shall be produced.

]]> http://mineralogy.frequentlyasked.info/2007/09/13/state-of-aggregation/feed/ http://mineralogy.frequentlyasked.info/2007/09/12/physical-properties-of-minerals/ http://mineralogy.frequentlyasked.info/2007/09/12/physical-properties-of-minerals/#comments Wed, 12 Sep 2007 19:43:26 +0000 admin http://mineralogy.frequentlyasked.info/2007/09/12/physical-properties-of-minerals/ Minerals possess certain physical properties that are considered in this chapter in the following order:

1. Certain characters depending upon light, such as color, luster, transparency, translucency, phosphorescence and fluorescence. Other optical properties especially valuable in the recognition of minerals in thin section under the microscope are dealt with in a later chapter.
2. Characters depending upon certain senses, such as those of taste, odor and feel.
3. Characters depending upon the state of aggregation, such as form, pseudomorphism, polymorphism, hardness, tenacity, fracture, cleavage, and surface tension effects. Crystallography, - the study of crystals, is considered in the next chapter.
4. The specific gravity of minerals.
5. Characters depending upon heat, such as fusibility.
6. Characters depending upon magnetism, electricity and radioactivity.
7. Color, Luster, Transparency, Etc.

Color

Color depends upon the absorption of some and the reflection of others of the colored rays or vibrations which compose ordinary white light. When a body reflects light to so small an extent as not to affect the eye, it appears black, but when it reflects all the vibrations of the different colors which compose white light, it appears white. Again, if it reflects the red vibrations of ordinary light and absorbs all the other vibrations, it appears red.

A blue mineral, such as sapphire, absorbs all the vibrations of white light with the exception of those that give the sensation of blueness to the eye.

The color of a mineral is often its most striking property. Unfortunately for purposes of identification, however, the colors of minerals vary very greatly. Even in the same species specimens are found having very different colors. The mineral quartz, composed of silicon dioxide, is commonly colorless or white, but it is also found with pinkish-yellow, green, brown, amethystine and even black colors. Corundum, composed of alumina, varies in color from pale brown to deep red and dark blue, the two latter varieties being the gemstones ruby and sapphire. The same crystal of a mineral may exhibit different colors, sometimes arranged in a regular fashion as in some crystals of tourmaline, at other times in patches as in certain specimens of fluor-spar, calcium fluoride.

The streak of a mineral is the color of its powder and may be quite different from that of the mineral in mass. For instance, black hematite gives a red powder. Streak is observed by producing a small quantity of the powdered mineral by scratching with a knife or file or by rubbing the mineral on a piece of unglazed porcelain or roughened glass called a streak-plate.

Some minerals, when turned about or looked at in different directions, display a changing series of prismatic colors, such as are seen in the rainbow or on looking through a glass prism. This is called a play of colors. It is shown by the diamond and is produced by the splitting-up of a ray of white light into its colored constituents as it enters and emerges from the mineral. Change ‘of color is a somewhat similar phenomenon extending over broader surfaces, the succession of colors being produced as the mineral is turned. This phenomenon is excellently displayed by certain varieties of the mineral feldspar, the colors shown including blues, greens, yellows and reds. Such a feldspar is an abundant constituent of a rock from southern Norway, and polished slabs of this rock in which the feldspar crystals lie in various directions are used for ornamental purposes. The change of color is caused by the interference of light reflected from thin plates of other minerals enclosed in parallel planes within the feldspar. Schiller, a nearly metallic luster shown by certain surfaces of the minerals hypersthene, schiller-spar, etc., is due to a somewhat similar cause. Reflection takes place either from minute plates arranged on parallel planes, or from cavities due to chemical action along certain parallel planes within the mineral.

Opalescence is a somewhat pearly or milky appearance shown by opal and moonstone. Iridescence is a display of prismatic colors due to the interference of rays of light in minute fissures which wall in thin films of air or liquid. These fissures are often the result of incipient fracture. Iridescence may sometimes be seen in quartz, calcite and mica. The brilliant display of colors given by the precious opal is due to the presence of very thin curved or distorted layers with slightly different optical properties.

Some minerals tarnish on the surface when exposed to the air and sometimes exhibit iridescent colors. This tarnish may result either from oxidation, or from the chemical action of sulphur and other elements which are generally present in the atmosphere in minute quantities. Tarnish may be distinguished from the true color by chipping or scratching the mineral, when the superficial nature of the tarnish is revealed. Copper pyrites often tarnishes to an iridescent mixture of colors. The mineral erubescite tarnishes readily on exposure to the air, and some varieties are called peacock ore.

Some crystals display different colors when viewed in different directions by transmitted light. This property, called pleochroism, ‘is considered with the special optical properties on a later page.

Luster

The luster of minerals differs both in intensity and kind, depending upon the amount and type of reflection of light that takes place at their surfaces.

There are six kinds of luster:

1. M Dallic - The ordinary luster of metals. When feebly displayed this luster is termed sub-metallic, Gold, iron pyrites and galena have a metallic luster; chromite and cuprite have a submetallic luster.
2. Vitreous - The luster of broken glass. When less well developed, it is called subvitreous luster. Quartz and rock-salt afford examples of vitreous luster, calcite of subvitreous.
3. Resinous - The luster of resin. Opal, amber and some kinds of zinc blended have a resinous luster.
4. Pearly - The luster of a pearl. It is shown by surfaces parallel to which the mineral is more or less separated into thin plates, reproducing to some extent the conditions of a pile of thin glass sheets, such as cover-glasses. Talc, brucite and selenite show pearly luster.
5. Silky - The luster of silk. This luster is peculiar to minerals having a fibrous structure. The fibrous form of gypsum known as satin-spar, and the variety of asbestos called amianthus are good examples of minerals having a silky luster.
6. Adamantine - The luster of a diamond.

The luster of minerals may be of different degrees of intensity, according to the amount of light reflected from their surfaces. Thus, when the surface of a mineral is sufficiently brilliant to reflect objects distinctly, as a mirror would do, it is said to be splendent. Certain varieties of hematite have a splendent luster. When the surface is less brilliant and objects are reflected indistinctly, it is described as shining. When the surface is still less brilliant and is incapable of giving any image, it is termed glistening, and glimmering denotes a still feebler luster. Minerals with no luster are described as dull.

As shown later, the various surfaces of a crystal may show different kinds and degrees of luster.

Transparency and Translucency

A mineral IS transparent when the outlines of objects seen through it appear sharp and distinct. Rock crystal-a variety of quartz-and selenite are good examples. Minerals are said to be sub transparent or semitransparent when objects seen through them appear indistinct. A mineral which, though capable of transmitting light, cannot be seen through is translucent. This condition is very common among minerals. When no light is transmitted the mineral is opaque, but it must be noted that this refers only to the appearance as usually seen. A large number of apparently opaque minerals become translucent when cut into very thin sections, and this property is of great importance, as shown in a later chapter, in the identification of minerals in rocks.

Many minerals which are opaque in the mass are translucent on the sharply broken edges and in splinters, as in the case of the common black flint from the Chalk of the south of England.

Phosphorescence and Fluorescence

Phosphorescence is the property possessed by some substances of emitting light after having been subjected to certain conditions such as heating, rubbing, or exposure to electric radiation or to ultra-violet light. Some varieties of fluorspar, when powdered and heated on an iron plate, display bright phosphorescence. Pieces of quartz when rubbed together in a dark room emit a phosphorescent light. Exposure to sunlight or even ordinary diffused light elicits phosphorescence from many minerals, as may be observed by transferring them rapidly to a dark room. Diamond, ruby and certain other minerals show brilliant phosphorescence after exposure to X-rays. Willemite, zinc orthosilicate, phosphoresces when exposed to X-rays, a fact employed to make certain that this mineral has been completely extracted from its ore.

Some minerals emit light whilst exposed to certain electrical radiations. This phenomenon is best exhibited by fluor-spar and for this reason is called fluorescence.

Taste, Odor and Feel

Taste

The characters of minerals dependent upon taste are only perceptible when the minerals are soluble in water. The following are terms used in this connexion: saline, the taste of common salt; alkaline, that of potash and soda; cooling, that of nitre or potassium chlorate; astringent, that of green vitriol; sweetish astringent, that of alum; bitter that of Epsom salts, and sour, that of sulphuric acid.

Odor

Some minerals have characteristic odors when struck, rubbed, breathed upon or heated. Terms used are:

• Alliaceous - the odor of garlic, given when arsenic compounds are heated.
• Horse-radish - the odor of decaying horse-radish, given when selenium compounds are heated.
• Sulphurous - the odor of burning sulphur, given off by pyrites when struck, or by many sulphides when heated.
• Fcetid - the odor of rotten eggs, given by heating or rubbing certain varieties of quartz or limestone.
• Argillaceous or Clayey - the odor of clay when breathed upon.

Feel

Smooth, greasy or unctuous, harsh, or meager or rough, are kinds of feel of minerals that may aid in their identification. Certain minerals adhere to the tongue.

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