Rocks

Microscopic characteristics

When dealing with new types or with rocks so fine grained that their component minerals cannot be determined with the aid of a hand lens, a petrographic microscope is used.

Separation of components

The division of the ingredients of a crushed rock powder from one to one more in order to get pure samples suitable for analysis is as well extensively practiced. It might be affected by way of a powerful electro magnet the strength of which can be keeping pace as desired. A weak magnetic field will attract magnetite, then haematite and other ores of iron. Silicates having iron will follow in exact order and biotite, enstatite, augite, hornblende, garnet, and similar ferro magnesian minerals may be consecutively abstracted, at last merely the colorless, non magnetic compounds, for instance muscovite, calcite, quartz and felspar, will stay. Chemical methods as well are useful. A weak acid will dissolve calcite from a crushed limestone, leaving only dolomite, silicates or quartz. Hydrofluoric acid will attack felspar before quartz, and if employed with vast concern will melt these and any glassy material in a rock powder before dissolving augite or hypersthene. Methods of separation by exact gravity have a still wider application. The simplest of these is levigation or treatment by a current of water, it is extensively employed in the mechanical analysis of soils and in the treatment of ores, but is not so victorious with rocks, as their components do not as a rule differ extremely greatly in exact gravity.

Fluids are used which do not attack the majority of the rock making minerals and at the similar time have a high specific gravity. Solutions of potassium mercuric iodide, cadmium borotungstate, methylene iodide, bromoform, or acetylene bromide are the principal media employed. They might be diluted to any preferred extent and again concentrated by evaporation. If the rock be an granite consisting of biotite, muscovite, quartz, oligoclase and orthoclase the crushed minerals will all float in methylene iodide like on gradual dilution with benzene they will be precipitated in the order given above. Although simple in theory these methods are tedious in practice, particularly as it is ordinary for one rock making mineral to surround another. However expert handling of fresh and appropriate rocks yields outstanding results.

Chemical analysis

In adding up to naked eye and petrographic microscopy investigations, chemical methods of research are of the greatest practical significance to the geographer. The crushed as well as separated powders, got by the procedure described above, might be analyzed and thus the chemical composition of the minerals in the rock determined qualitatively or quantitatively. The chemical testing of petrographic microscopy sections as well as minute grains by the help of the petrographic microscope is a extremely stylish and valuable means of discriminating between the mineral components of fine grained rocks. Thus the attendance of apatite in rock sections is recognized by covering a bare rock section with solution of ammonium molybdate like a turbid yellow impulsive form over the crystals of the mineral in question. A lot of silicates are insoluble in acids and cannot be tested in this way, but others are partly dissolved, leaving a film of gelatinous silica which can be stained with coloring matters such as the aniline dyes.

Complete chemical analyses of rocks are as well widely made use of as well as are of the first significance, particularly when new species are beneath description. Rock analysis has of late years reached a high pitch of modification and difficulty. As a lot of as twenty or twenty five components might be strong minded, although for practical purposes a knowledge of the relative proportions of silica, alumina, ferrous and ferric oxides, magnesia, lime, potash, soda and water will carry us a long way in determining the position to which a rock is to be assigned in any of the conventional classifications. A chemical analysis is in itself typically enough to point to whether a rock is igneous or sedimentary and in both case to show with substantial accuracy to what subdivision of these classes it belongs. In the case of metamorphic rocks it often set up whether the original mass was sediment or of volcanic origin.