Mineralogy

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.