The amphiboles are a large group of minerals, the silicates of many different bases, the commonest being alumina, iron oxide, lime, magnesia and the alkalis. They are constituents of many crystalline igneous rocks and of metamorphic schists. In many of their properties they closely resemble the pyroxenes. They occur generally in black or dark green crystals, usually long, narrow and blade-like, and owing to their perfect cleavages their surfaces are smooth and bright. The commoner varieties are hornblende, actinolite and tremolite. Research Amphibole
Aphanite is a very compact, dark-coloured rock, consisting of hornblende, or pyroxene, and feldspar, but neither of them in perceptible grains. Research Aphanite
Crocidolite is a mineral occurring in silky fibres of a lavender blue colour. It is related to hornblende and is essentially a silicate of iron and soda. A silicified form, in which the fibres penetrating quartz are changed to oxide of iron, is the yellow brown tiger's-eye of the jewellers. Research Crocidolite
Diorites are a group of igneous rocks composed essentially of a soda-lime feldspar and hornblende, embracing a wide range of types from acid to basic. Diorites were formed by cooling far below the surface and occur in the Scottish Highlands, the Channel Islands, California and other places. Research Diorites
In geology, the term foliation refers to the property, possessed by some crystalline rocks, of dividing into plates or slabs, which is due to the cleavage structure of one of the constituents, as mica or hornblende. It may sometimes include slatey structure or cleavage, though the latter is usually independent of any mineral constituent, and transverse to the bedding, it having been produced by pressure. Research Foliation
Gneiss refers to a crystalline rock, consisting, like granite, of quartz, feldspar, and mica, but having these materials, especially the mica, arranged in planes, so that it breaks rather easily into coarse slabs or flags. Hornblende sometimes takes the place of the mica, and it is then called hornblendic or syenitic gneiss. Similar varieties of related rocks are also called gneiss. Research Gneiss
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
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Amphibole
