Rock minerals. Their chemical and physical characters and their determinayion in thin section / Минералы. Их химические и физические характеристики и определение в шлифе

The term Rock Minerals is sufficiently indefinite to permit of considerable latitude in the choice of minerals to be included by it. Besides those that constitute the mass of any rock there are the less abundant though common kinds, as well as occasional, exceptional, ones. In any case the mineral may be an original constituent of the rock or one that has been developed in it subsequent to its formation. Rock Minerals, therefore, embrace not only all primary minerals, but all those of secondary origin produced by any manner of alteration of previously existing minerals. Among such secondary minerals are some that are better known as fillings of cavities and veins in rocks than as rock constituents, such as the zeolites. These, however, also occur as replacements of primary crystals within the body of rocks, and so constitute rock minerals in a narrower sense. The term becomes somewhat more comprehensive when it is applied to minerals formed in pegmatite veins, which are undoubtedly rocks of intrusive, igneous, origin, for there are many uncommon minerals well developed in such veins that are not known to crystallize within the main mass of larger rock bodies. However, it is customary to exclude certain classes of veins of minerals, such as ore veins, from the category of rocks, though their mode of formation in some instances is the same as that of many kinds of secondary rock minerals, and their study by petrographical methods in thin sections has been undertaken more or less systematically by several investigators. The inclusion of such vein minerals within the field of the petrographer would place almost the whole range of mineralogy before him and cause a distinction between the petrographer and mineralogist to consist, as it virtually does at present, in the objects and methods of research rather than in the materials investigated <...>