Feldspar or felspar is a name given to a group of minerals, closely related in crystalline form, and all silicates of alumina with either potash, soda, lime, or, in one case, baryta. They occur in crystals and crystalline masses, vitreous in lustre, and breaking rather easily in two directions at right angles to each other, or nearly so. The colours are usually white or nearly white, flesh- red, bluish, or greenish. The group includes the monoclinic (orthoclastic) species orthoclase or common potash feldspar, and the rare hyalophane or baryta feldspar; also the triclinic species (called in general plagioclase) microcline, like orthoclase a potash feldspar; anorthite or lime feldspar; albite or soda feldspar; also intermediate between the last two species, labradorite, andesine, oligoclase, containing both lime and soda in varying amounts. The feldspars are essential constituents of nearly all crystalline rocks, as granite, gneiss, mica, slate, most kinds of basalt and trachyte, etc. The decomposition of feldspar has yielded a large part of
the clay of the soil, also the mineral kaolin, an essential material in the making of fine pottery. Common feldspar is itself largely used for the same purpose. Research Feldspar
Felsite or felstone is a hard, compact, volcanic, fine grained rock, flint-like in fracture, consisting essentially of orthoclasefeldspar with occasional grains of quartz occurring in the older geological formations. Research Felsite
Granite is a plutonic, igneous, crystalline, granular rock, consisting generally of quartz, feldspar, and mica, mixed up without regular arrangement of the crystals, and usually of a whitish, greyish, or flesh-red colour. The grains vary in size from that of a pin's head to a mass of almost one meter, but they seldom exceed about two centimetres. When they are of this size, or larger, the granite is said to be 'coarse-grained.'.
Granite is one of the most abundant of the igneous rocks seen at or near the surface of the earth, and was formerly considered as the foundation rock of the globe, or that upon which all sedimentary rocks repose; but it is now known to belong to various ages from the Pre-Cambrian to the Tertiary, the Alps of Europe containing granite of the later age. In Alpine situations it presents the appearance of having broken through the more superficial strata; the beds of other rocks in the vicinity rising towards it at increasing angles of elevation as they approach it. It forms some of the most lofty of the mountain chains of the eastern continent, and the central parts of the principal mountain ranges of Scandinavia, the Alps, the Pyrenees, and the CarpathianMountains are of this rock.
Granite supplies one of the most durable materials for building, as many of the ancient Egyptian monuments testify. It varies much in hardness as well as in colour, in accordance with the nature and proportion of its constituent parts, so that there is much room for care and taste in its selection. Granite in which feldspar predominates is not well adapted for buildings, as it cracks and crumbles down in a few years. The Aberdeen bluish-gray granite is celebrated for its great durability, and also for its beauty. The Peterhead red granite, the hue of which is due to its feldspar being the flesh-coloured potash variety called orthoclase, is highly esteemed for polished work, as columns, pillars, graveyard monuments, etc. Granite in which mica is replaced by hornblende is called syenite; when both mica, and hornblende are present it is called syenitic granite; when talc supplants mica it is called protogene, talcose, or chloritic granite; a mixture of quartz and hypersthene, with scattered flakes of mica, is called hypersthenic granite; and the name of graphic granite, or pegmatite, is given to a variety composed of feldspar and quartz, with a little white mica, so arranged as to produce an irregular laminar structure. When a section of this latter mineral is made at right angles to the alternations of the constituent materials, broken lines resembling Hebrew characters present themselves; hence the name.
Granite abounds in crystallized earthy minerals; and these occur for the most part in veins traversing the mass of the rock. Of these minerals beryl, garnet, and tourmaline are the most abundant. It is not rich in metallic ores. The orientalbasalt, found in rolled masses in the deserts of Egypt, and of which the Egyptians made their statues, is a true granite, its black colour being caused by the presence of hornblende and the black shade of the mica. The oriental red granite chiefly found in Egypt, and of which Pompey's Pillar and Cleopatra's Needles were constructed, is composed of large grains or imperfectly formed crystals of flesh-coloured feldspar, of transparentquartz, and of black hornblende. Research Granite
Hardness refers to the resistance of a smooth surface of a rock or mineral to scratching. It is determined by the binding force of atoms within the crystal structure and is measured according to Moh's ten division scale of hardness, ranging from 1 the softest, to 10 the hardest: 1) talc 2) gypsum 3) calcite 4) fluorite 5) apatite 6) orthoclase 7) quartz 8) topaz 9) corundum 10) diamond.
Materials, according to this arrangement, which are scratched by quartz and are not scratched by felspar are said to have a hardness between 6 and 7.
Igneous rock is rock that has originated from molten or semi-molten magma. Igneous rocks are composed almost entirely of silicate minerals. Of many different types, they can be classified in various ways: by composition, crystal size, or mode of occurrence. One method is to divide them according to their silica content into three main groups: acid rocks, basic rocks, and intermediate rocks. There is, however, continuous gradation from acid to basic. Acid rocks are characterized by the presence of 10 per cent or more of quartz, usually accompanied by lighter- coloured minerals such as orthoclasefeldspar and muscovite. This acid group includes rocks such as granite and rhyolite. Molten acidlava is very viscous, and its viscosity prevents the molten lava flowing very far and produces tall, conical volcanoes of the Vesuvius type. Intermediate rocks have less than 10 per centquartz; diorite and andesite are typical of this group.
Basic igneous rocks are characterized by the absence of quartz and the predominance of dark minerals such as ferromagnesian minerals (amphiboles, pyroxenes). Common basic igneous rocks include gabbros and basalts. When extruded on to the Earth's surface these low-viscosity basic lavas form flat plateaux such as the ColumbiaPlateau, USA. Those igneous rocks that crystallize below the Earth's surface are termed intrusive or plutonic, according to whether they were formed at shallow depths, like dolerite, or deeper down, like granite. All these rocks have relatively large crystals produced by slow cooling of the molten magma. Igneous rocks that are extruded at the Earth's surface are termed extrusive or volcanic: basalt is an example. The rapid cooling that occurs at the surface does not allow large crystals to form, and these rocks are glassy or contain only small crystals. Research Igneous Rock
Microcline is a mineral with the formulae KAlSi3O8 and a relative hardness of 7. It has the same composition as orthoclase but is distinguished by triclinic twinning (usually requiring a microscope to detect). If a feldspar is a deep green it is microcline - and sometimes called 'amazon stone' or amazonite after the Amazon river. It is recognisable from plagioclase by a lack of striations. Research Microcline
Orthoclase has the formulae KAlSi3O8 and a relative hardness of 6. It is used in the manufacture of porcelain and for other industrial purposes. Research Orthoclase
Pegmatite is a variety of extremely coarse-grained igneous rock chemically similar to and closely associated with granite. The mineral constituents of
pegmatite are largely those typical of acidic intrusive rocks: orthoclasefeldspar, quartz, and mica. Individual mineral crystals may be up to several meters in length. The growth of crystals this large indicates that the parent magma, from which the rock solidified, was able to cool very slowly, after injection into fissures extending outward from a central magma chamber.
Pegmatite is widely distributed in the crust of the earth but is found especially in older mountain chains, where it is restricted essentially to those surface areas in which igneous and metamorphic rocks are prevalent. Minerals containing the elements boron, beryllium, and lithium are abundant in some pegmatites. Other pegmatites contain commercial quantities of feldspar, mica, and gem-grade tourmaline. Research Pegmatite
Abbreviations
Research Anorthoclase
