Mineralogy

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.