Aegirite (Aegirine) is a rock-forming mineral of the pyroxenes group, distinguished by it's long needle-like crystals, found mainly in rocks rich in soda and poor in silica. Named after Aegir, Icelandic god of the sea. It has the formulae NaFe(Si2O6) and a relative hardness of 7. Research Aegirite
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
Diabase is a dark, intrusive, basic igneous rock made up of plagioclasefeldspar crystals, surrounded by smaller grains of pyroxenes, such as augite and up to ten percent quartz. Diabase is formed from magna that has cooled just below the Earth's surface, and is sometimes found as intrusions in older rock. A common place to find diabase is in the neck of old volcanoes, where it has formed a plug. Research Diabase
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
Jade is a compact, opaque gemstone ranging in colour from dark green to almost white. The term is applied to specimens cut from the minerals jadeite and nephrite.
Jadeite, the less common and more highly prized of the two minerals, is a silicate of sodium and aluminium, NaAl(SiO3)2, usually containing some iron, calcium, and magnesium. It belongs to the group of minerals called pyroxenes.
Jadeite crystallizes in the monoclinic system but rarely occurs in distinct crystals and is usually found in fibrous, compact, massive aggregates. It has a hardness ranging from 6.5 to 7; it is extremely tough and difficult to break. The lustre on fresh fracture is dull and wax like, but polished jadeite has a vitreouslustre. Jadeite is found chiefly in eastern Asia in Burma, as well as in sections of Tibet and southern China. Nephrite, a member of the amphibole group of minerals, is a silicate of calcium and magnesium, with a small amount of iron replacing part of the magnesium. It is a tough, compact variety of the mineral tremolite with a hardness of 6 to 6.5. Polished nephrite has an oily lustre. It is found in Alaska, Mexico, New Zealand, Siberia, and Turkestan.
Jade was used in ancient times for weapons, utensils, and ornaments. A variety of jade called axstone is used by the natives of the South Sea islands for making hatchets. Jade has always been prized by the Chinese and Japanese as the most precious of all stones, and the most beautiful specimens of carved jade in the form of ornamental pieces, such as vases, bowls, tablets, and statues, many of which are now museum pieces, were made in China. Jade is a highly valued gemstone used in jewellery. Research Jade
Pyroxenes refers to a group of closely related and dark coloured rock forming minerals. Examples are aegirine, diopside, enstatite, hedenbergite, jadeite and spodumene. Pyroxenes are common rock-forming minerals which are found in most igneous and many metamorphic rocks. Their presence in a rock indicates that it crystallised at a high temperature. Research Pyroxenes
Pyroxenite is a medium to coarse-grained intrusive igneous plutonic rock made up almost completely of pyroxenes. Containing less than 45 percent silica,
pyroxenite contains large quantities of iron- and magnesium-bearing minerals. Research Pyroxenite
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Aegirite
