Page:EB1911 - Volume 18.djvu/537

 methods which have been used: simple sublimation (e.g. arsenolite); interaction of gases (e.g. haematite, from steam and ferric chloride; cassiterite, from steam and stannic chloride or fluoride); action of gases on liquids and solids; slow cooling of fused masses, either with or without the presence of agents rninéralisateurs (e.g. minerals in furnace slags); from aqueous solution sometimes at a high temperature and under pressure (e.g., quartz); electrolysis; or even by subjecting dry amorphous material to enormous pressure. The chemical reactions by which Various minerals have been obtained are often of considerable help in speculating as to their mode of origin in nature, though it must be born in mind that the same mineral may have been formed, both naturally and artificially, by more methods than one. In this direction important results have been obtained experimentally by J. H. van’t Hoff and his pupils on the formation of oceanic salt deposits, and by J. H. L. Vogt with slags. Many minerals used as gem-stones have been prepared artificially, e.g. diamond and ruby (see : Artificial).

While some minerals are of rare and sporadic occurrence in rock-cavities and mineral-veins, others are widely distributed as important constituents of rocks. The same mineral species may have several distinct modes of occurrence and origin, and be associated with different minerals in each case; facts which are well illustrated by (q.v.).

Minerals of Igneous Rocks.—The rock-forming minerals of primary origin in igneous rocks have crystallized out from the magma, or fused silicate-mass, which on consolidation gave rise to the rock-mass. Magmas sometimes contain a considerable amount of water and are then in a state of aqueo-igneous fusion, rather than of dry fusion: in such cases very coarsely crystalline rocks (pegmatites) often result, and under these conditions minerals of many kinds are formed as well-developed crystals. Those minerals which are present in large amount in igneous rocks are distinguished as essential constituents, since it is on these that the classification of igneous rocks is largely based: the most important are quartz, felspars, pyroxenes, amphiboles, micas and olivines. Felspars of different composition are present in almost all kinds of igneous rocks, while quartz and olivine are characteristic of acid (e.g. granite, rhyolite) and basic (e.g. basalt, peridotite) rocks respectively. When the magma contains alkalies in relatively large amount the “felspathoid” minerals, nepheline and leucite, are formed (e.g. in nepheline-syenite, leucite-basalt, &c.). Other minerals occurring as primary constituents, but only in small amounts, are distinguished as accessory; thus small crystals of magnetite, apatite, zircon, &c., are of frequent occurrence disseminated in igneous rocks (see ). Sometimes these accessory constituents are concentrated by magmatic differentiation, important ore-deposits sometimes resulting in this manner (e.g. of chromite, or nickel-bearing pyrrhotite). The alteration of igneous rocks by weathering and other processes results in the alteration of some or all of the primary minerals with the production of others, which are spoken of as secondary minerals: thus felspars are often partly or wholly altered to kaolin, olivine to serpentine, pyroxene and mica to epidote, chlorite, &c.

Minerals are also formed by the vapours given off by igneous magmas. The gases emitted by volcanoes and solfataras may deposit directly by sublimation, or by their chemical interaction, such minerals as sulphur, sal-ammoniac, haematite, which occur, for instance, as incrustations on Vesuvian lava: the boric acid of the Tuscan lagoons has also originated in this way. The effects produced by the exhalations of deep-seated magmas are more complex in character, since the vapours, being more confined, have more opportunity of acting chemically not only on the surrounding rocks but also on the igneous rock-mass itself before its final consolidation. A good example of the “pneumatolytic” action produced by the vapours from a mass of granitic magma is afforded by veins of tin-ore, in which the ore (cassiterite) is associated with minerals containing boron and fluorine, such as topaz, tourmaline. lepidolite, fluor-apatite

and fluor-spar. The production of such minerals may be accounted for by assuming the presence of stannic fluoride in the vapours, which by reacting on water vapour would deposit cassiterite with the liberation of hydrofluoric acid, and this would again react on other minerals. The topaz and tourmaline crystals often found in the cavities of granites and pegmatites have doubtless been formed in this manner. In a similar way the exhalations of basic magmas have given rise to chlor-apatite with associated sphene and ilmenite, as, for example, in the extensive apatite veins in Connexion with gabbro in southern Norway.

Minerals of Metamorphic Rocks.—By the baking action of a deep-seated igneous mass on the surrounding rocks or on included rock-fragments, various new minerals are developed. By this process of thermal or contact-metamorphism well-crystallized examples of many minerals have often been formed; e.g. in calcareous rocks (limestones), especially those containing some magnesia and silica, vesuvianite, garnet, diopside, tremolite, wollastonite, &c., are developed; in argillaceous rocks (slates), chiastolite and staurolite are characteristic products; and in arenaceous rocks (sandstones), cordierite and sillimanite often result. The effects of pressure (dynamo-metamorphism) on rocks of various kinds, especially those of igneous origin, also result in the production of new minerals: e.g. pyroxene is transformed to amphibole, orthoclase to Muscovite, plagioclase to zoisite, olivine to tremolite, &c. In gneisses and crystalline schists, quartz, felspar, mica, talc, amphibole, &c. are important constituents.

Minerals of Sedimentary Rocks.—By the weathering and disintegration of igneous and metamorphic rocks the various minerals set free and the products of decomposition of others supply the material of sedimentary rocks; thus sandstones consist largely of quartz, shales of kaolin and other clay minerals. Those minerals (e.g. gem-stones and gold) which resist the action of weathering processes are found as water-worn pebbles and grains in detrital deposits. Other sedimentary rocks consist of minerals deposited from solution either by chemical or organic agencies, from sea-water, lakes or springs: e.g. the calcite of limestones, deposits of bog-iron-ore (limonite), gypsum, rocksalt, &c.

Minerals Segregated in Veins and Rock-cavities.—Water percolating through rock-masses takes up mineral matter in solution, and the solutions so formed may further react on the minerals composing the rocks. Such solutions will deposit some of their dissolved material in rock-cavities with the production of various minerals., For instance, the amygdaloidal cavities of basic volcanic rocks (e.g. basalt, melaphyre), especially when the rocks are somewhat weathered, are frequently partly or completely filled with agate or beautifully crystallized zeolites, calcite, &c. The crevices and joint-planes of limestone become in this way coated with crystals of calcite, and those of siliceous rocks with quartz, giving rise to the abundantly occurring quartz-veins. In sedimentary rocks, pyrites, flint and other minerals become segregated round a nucleus of organic matter. The beautiful crystal-lined crevices in the crystalline rocks of the Alps have much the same origin, and so have the various types of ore-deposits, including metalliferous veins or lodes. In the latter cases, however, the solutions are no doubt sometimes of deep-seated origin and often connected with igneous and metamorphic processes. Metalliferous veins are storehouses of crystallized minerals of almost every kind, some being the ores themselves and others, such as quartz, calcite, barytes, fluorspar, being gangue minerals. By the weathering of the metallic minerals of mineral-veins numerous other finely crystallized minerals result: for example, in the upper oxidized portion of Veins of lead-ore (galena), crystals of anglesite, cerussite and pyromorphite are often met with; in veins of copper-ore the alteration of chalcopyrite gives rise to malachite, chessylite and cuprite.

Alteration of Minerals: Pseudomorphs.—Crystals which have been formed under one. set of conditions of temperature and pressure and in the presence of certain solutions, will in many