Micro-chemical Methods.—Chemical methods are frequently of great use. Weak hydrochloric acid will dissolve calcite from a crushed limestone leaving dolomite and silicate minerals, and hydrofluoric acid may be used to extract the glassy base of vol canic and hypabyssal rocks from the pyroxenic minerals. Of much wider use however are microchemical methods for distinguishing minerals under the microscope. They are of special value in dis criminating alkali minerals like nepheline, haiiyne, nosean, analcime and sodalite in the ground mass of igneous rocks. The methods depend on selective attack by acids, the formation of distinctive precipitates and the staining of gelatinous residues. Quartz and felspar in fine grained rocks may be differentiated by etching with hydrofluoric acid; quartz remains clear while the surface of the felspar grain becomes cloudy, being altered to aluminium fluosilicate, and may be stained by an aniline dye. This method is particularly useful in discriminating quartz and felspar in fine grained metamorphic rocks in which the felspar is without crystal form, is untwinned and shows lack of cleavage.
Chemical Analysis.—The ultimate chemical composition of a rock is of fundamental importance in determining its nature, and the chemical analysis of rocks is now widely carried out. In rock analysis of the better class as many as 25 constituents are determined and recorded. The most abundant, or major, constit
uents are stated as oxides, and any rock analysis of good quality records the content of at least nine or ten of these. They are silica, alumina, ferric and ferrous oxides, mag nesia, lime, soda, potash and water. These nine oxides make up about 98% of the igneous rocks. Two other oxides are usually determined, and P205, as they are represented in the common minor constituents—ilmenite and apatite—titanium being also present in some pyroxenes, amphiboles and micas. Such a chemi cal analysis is usually sufficient to determine to which of the two great classes, igneous or sedimentary, a rock belongs, and if it is igneous, the position the rock is to be assigned in any of the conventional classifications. In the case of metamorphic rocks it often establishes whether the original mass was a sediment or of igneous origin.
Specific Gravity.—By the balance and the pycnometer the specific gravity of rocks is determined in the usual way. It is greatest in those rocks rich in iron, magnesia and the heavy metals, and least in rocks rich in alkalis, silica and water. Eclogites (sp.gr. 3.5) are among the heaviest of the silicate igneous rocks, while the vitreous lavas of high silicity like obsidian (sp.gr. 2.35) are among the lightest.