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Ice spar is a variety of feldspar, the crystals of which are very clear like ice.
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Iceland spar is a transparent variety of calcite, the best of which is obtained in Iceland. It is used for the prisms of the polariscope, because of its strong double refraction.
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Idrialine is a bituminous substance obtained from the mercury mines of Idria, where it occurs mixed with cinnabar.
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Igneous intrusion refers to a body of igneous rock that has made its way into pre-existing rock (known as country rock). Igneous intrusions are emplaced as magma, which is less dense than solid rock and therefore tends to move upwards. It can then force its way through cracks in the rocks and can wedge them apart or, if it is hot enough, it can melt and replace them.
Igneous intrusions can be of a variety of shapes and sizes, ranging from huge batholiths to bodies only one or two meters across. The general term 'pluton' can be applied to any of these. Intrusions may cut across the bedding of the country rock. They are then termed 'discordant' or ' transgressive'; dykes and laccoliths are examples. Intrusions that follow the bedding of the country rock, such as sills, are termed 'concordant'.
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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 orthoclase feldspar and muscovite. This acid group includes rocks such as granite and rhyolite. Molten acid lava 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 cent quartz; 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 Columbia Plateau, 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.
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Ilmenite or titaniferous iron oxide is an iron-black or steel-grey coloured common rock-forming mineral which is chiefly found in small crystals in basic igneous rocks and schists, consisting chiefly of the oxides of iron and titanium. It has weak magnetism and a relative hardness of six. It is the major ore of titanium. It is also known as menaccanite after Menaccan, in Cornwall, where it was first found.
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Ilvaite (yenite or jenite) is a silicate of iron and lime occurring in black prismatic crystals and columnar masses.
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In mineralogy, an inclusion is a foreign substance, either liquid or solid, usually of minute size, enclosed in the mass of a mineral.
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Indicolite is an indigo-blue coloured variety of tourmaline.
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Inesite is a secondary mineral of a fleshy pink colour formed in zones of metamorphism and by metasomatism. Inesiteis a hydrous hydrated silicate of calcium, manganese and iron and was confirmed as a distinct species of mineral in 1888, though in 1940 it was suggested that inesite was in fact a calcium-rich variety of rhodonite. Inesite crystals are triclinic, they are prismatic and commonly have a slanted or chisel-shaped termination.
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Intermediate rock refers to an igneous rock that is transitional between acidic and basic rocks. Have a silica content between 54-65%.
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Iodyrite is a yellowish coloured mineral form of Silver iodide.
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Iolite or dichroite is a silicate of alumina, iron, and magnesia, having a bright blue colour and vitreous lustre. It is remarkable for its dichroism.
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Iridosmine or iridosmium is the native compound of iridium and osmium. It is found in flattened metallic grains of extreme hardness, and is often used for pointing gold pen nibs.
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Iron has the formulae Fe and a relative hardness of 5. It is recovered primarily from iron bearing minerals and is recognizable by the oxide coating on its surface (rust).
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Iron clay is a yellowish clay containing a large proportion of an ore of iron.
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Iron flint is an opaque, flint-like, ferruginous variety of quartz.
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Ironstone is a rock containing at least fifteen percent iron. Ironstone forms in igneous rocks through mahmatic differentiation - dense heavy minerals such as magnetite crystallise first and sink to the base of the molten magma to form an iron-rich layer. Ironstone also forms in sedimentary rocks such as limestone and sandstone.
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Isinglass is a popular name for mica, especially when in thin sheets.
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Isometric refers to cubic. Three axis, all the same length and at right angles to each other.
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Isostasy is a geological theory explaining the general equilibrium in the earth's crust, which is supposed to be maintained by the yielding or flow of rock material beneath the surface under gravitative stress. By the theory of
isostasy each unit column of the earth, from surface to centre, has approximately the same weight, and the continents stand higher than the ocean beds chiefly because the material of the crust has there less density.
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Itacolumite is a laminated, granular, siliceous rocks, often occurring in regions where the diamond is found.
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Abbreviations
Rocks & Minerals
