Petrology

Microscopic Characters.

But when dealing with unfamiliar types or with rocks so fine grained that their component minerals cannot be determined with the aid of a lens, the geologist is obliged to have recourse to more delicate and searching methods of investigation. With the aid of the blowpipe (to test the fusi bility of detached crystals), the goniometer, the magnet, the magnifying glass and the specific gravity balance the earlier travellers attained surprisingly accurate results. Examples of these may be found in the works of von Buch, Scrope, Darwin and many others. About the end of the 18th century, Dolomieu examined crushed rock powders under the microscope and Cordier in 1815 crushed, levigated and investigated the finer ground-mass of igneous rocks. His researches are models of scrupulous accuracy, and he was able to announce that they consisted essen tially of such minerals as felspar, augite, iron ores and volcanic glass, and did not differ in nature from the coarser grained rocks. Nicol, whose name is associated with the discovery of the Nicol prism, seems to have been the first to prepare thin slices of mineral substances, and his methods were applied by Witham (1831) to the study of plant petrifactions This method, subsequently of such far-reaching importance in petrology, was not at once made use of for the systematic in vestigation of rocks, and it was not till 1858 that Sorby pointed out its value. The great work of this investigator was the produc tion of very thin sections of rocks and their systematic examina tion under the high powers of the microscope. To-day the densest, blackest rock can be made to yield a section of in. in thick ness, so thin and transparent that fine print can be easily read through it, and transmitting light so clearly that the most high powered objectives of the microscope can be used to discern and study the minutest structures it presents with the same facility that they can be employed upon sections of organic material prepared by the microtome. The introduction of this powerful weapon of research, inaugurated a new era in petrology—that of petrography or the descriptive branch of petrology. The optical study of sections of crystals had been advanced by Sir David Brewster, Nicol and other physicists and mineralogists, and it only remained to apply their methods to the minerals visible in rock sections. The pioneer workers who brought quantitative methods into the optical side of the problem were Des Cloiseaux, Rosenbusch, Zirkel, Schuster, Fouque and Levy. At the present day optical methods for determining minerals with the petro graphic microscope are highly developed.

Although rocks are now studied principally in micro-sections the investigation of fine crushed rock powders, which was the first branch of microscopic petrology to receive attention is still in use. A mineral whose optical properties are known can be accurately determined by immersing its powder in liquid media whose indices of refraction are known, and determining the optical constants. In the hands of a skilled petrographer the principal optical constants of a single grain of a mineral can be determined in half an hour. The chief fundamental constants measured are the principal indices of refraction, the crystallographic orientation of the directions of light-vibration corresponding to those indices, and the amount of absorption of light vibrating in these directions, all for one or more standard wave-lengths of light. The double refraction, optical character, optical axial angle, dispersion of the optic axes and bisectrices, extinction angle and pleochroism are all fixed by the fundamental constants and can be estimated under the microscope.

The immersion media cover a range of optical refraction from that of water (1.333) to that of a selenium—arsenic selenide melt. (3.17). The liquids are chiefly organic substances,—acetone alco hol, cinnamon oil, monobrom-naphthalene and methylene iodide. Most minerals have refraction constants ranging in the interval 1.450-1.870, and their refractive indices may therefore be matched by a set of liquids whose refractions cover this range. The modern rotation apparatus whereby thin sections may be tilted from the horizontal so that the axis of the microscope passes through them at different angles has been of inestimable value in measuring accurately the optical constants of a single grain in a rock slice, and is now widely used for the determination of felspar and other mineral species of variable composition. A few measurements on a selected section of a mineral permit the determination of its exact chemical composition and the crystallographic orientation of the section itself. The technical methods employed for the determinations referred to above are largely founded on the use of polarized light.

Mechanical Separation of Rock Constituents.

The sepa ration of the constituents of a crushed powder in order to obtain pure samples suitable for analysis is also extensively practised. The two principal methods adopted involve either the electro magnet or heavy liquids.

By the use of an electromagnet the component minerals of a rock may be separated by varying the strength of the current. A weak magnetic field attracts magnetite, then hematite and ilmenite. Silicates containing iron will follow in definite order, augite, hornblende, tourmaline, olivine and biotite being successively attracted. The degree of attraction is not however proportional to the iron content. The colourless non-magnetic minerals—quartz, felspar, muscovite, nepheline, etc.—remain.

Separation by means of water is not much used in petrographic work. However, a preliminary "panning" as practised by miners is often useful as a means of concentrating rare minerals occurring in very small quantity in the rock before the application of the methods next to be described. Methods of separation of minerals by which they are sorted according to their specific gravities by means of heavy fluids have an extensive application. The fluids used are those which do not attack the majority of the rock forming minerals. Of the many liquids used methylene iodide (sp.gr. 3.32) and bromoform (sp.gr. 2.86) are perhaps the most convenient. For more dense minerals Clerici's solution—an aque ous solution of thallium formate—is best adopted. By concentra tion the specific gravity can be raised to 4.32 at 60° C so that even the heavier rock minerals can be separated without difficulty. By dilution with water, or, in the case of methylene iodide, dilu tion with bromoform or benzol, successive crops of minerals may be separated in appropriate separating funnels. In this way a granite may be successively fractionated into its component minerals, biotite (sp.gr. 3.1), muscovite (2.85), quartz (2.65), oligoclase (2.64) and orthoclase (2.56). All these minerals float in methylene iodide : on dilution with benzol they are precipitated in the order given. Rocks like eclogite containing heavier minerals, such as ilmenite (sp.gr. garnet (4-20) titanite (3.50) and diopside (3.29), may be similarly separated by means of a thal lium formate-water solution.