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A Mineral Environment is the rock (or rock type) in which a mineral or a group of associated minerals forms and occurs. Mineral environments include igneous, sedimentary, and metamorphic rocks as well as several types of veins and replacement deposits. These are outlined more specifically below.
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Acidic rock |
A type of igneous rock (e.g. granite) that consists predominantly of light colored mineral and more than 66% free of combined silica (see also Basic rock, Intermediate rock, and Ultrabasic rock). |
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Alkaline rock |
A rock containing more than average amounts of potassium and sodium bearing minerals. |
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Amygdaloidal rock |
A volcanic rock containing numerous gas cavities (amygdules) filled with such secondary minerals as calcite, quartz, and zeolites. |
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Basic rock |
An igneous rock (e.g. gabbro) with low silica content and a high percentage of pyroxene, hornblende, and labradorite. (See also Acidic rock, Intermediate rock, Ultrabasic rock). |
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Batholith |
A huge body of plutonic rock that has been intruded deep into the earth's crust and later exposed by erosion. |
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Bituminous rocks |
Rocks that contain (and sometimes smell of) asphalt, tar, or petroleum. |
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Carbonaceous |
Conposed largely of organic carbon (i.e., carbon derived from plant and animal tissue). |
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Cataclastic metamorphism |
A metamorphism due principally to directed pressure and resulting in rocks with cataclastic texture. |
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Cataclastic rock |
A metamorphic rock produced by the crushing and grinding of preexisting rocks, which are still visible as crushed and flatttened minerals and as angular fragments (from the Greek klastos, "broken"). |
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Chemical sedimentary rock |
A rock formed by chemical process; gypsum is a chemical sedimentary rock formed by chemical precipitation. |
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Clastic rock |
A sedimentary rock that is made up of fragments of preexisting rocks, transported mechanically into the place of deposition. |
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Clay |
Any soft sediment or deposit that is plastic when wet and consists of very fine grained, micalike materials, mainly hydrous aluminum silicates |
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Contact metamorphism |
Metamorphism directly related to the intrusion of magmas and taking place at or near the contact with the molten rock. |
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Detrital sediment |
A deposit of mineral and rock fragments that have been transported to their place of deposition. |
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Dynamothermal metamorphism |
Metamorphism resulting from the combined effects of heat and pressure; also called regional metamorphism. |
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Epithermal vein |
A vein formed at shallow depths from ascending hot solutions. |
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Extrusive rock |
An igneous rock that solidifies on the surface of the earth. |
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Feldspathic rock |
A rock that contains feldspar as a principal constituent. |
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Fragmental rock |
Sedimentary rock consisting of rock and mineral fragments. |
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Gneissose rock |
A rock that has the banded appearance of a gneiss but is not formed by metamorphism. |
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Hydrothermal alteration |
An alteration of minerals or rocks by the action of superheated mineral rich fluids, usually water that has been heated to very high temperatures within a crystallizing magma. |
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Hydrothermal metamorphism |
Changes in the structure or composition of rock, caused by the action of hydrothermal fluids. |
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Hydrothermal replacement |
A change in a rock or mineral deposit due to the addition or removal of minerals by hydrothermal fluids. |
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Hypothermal vein |
A vein formed at relatively great depth and at relatively high temperatures (300 - 500 deg. C). |
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Igneous rock |
Rock formed by the solidification of magma. |
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Intermediate rock |
Igneous rock (e.g. syenite or diorite) that is transitional between acidic and basic rock, having a silica content of between 54% and 65% (see Acidic rock, Basic rock, and Ultrabasic rock). |
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Intrusive rock |
An igneous rock that formed underground, from magma that was squeezed into cracks or crevices, or between layers of older rocks. |
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Laccolith |
A lens shaped body of igneous rock with a dome shaped upper surface and a flat bottom surface, and with both surfaces parallel to the beddding or foliation of the enclosing rocks. |
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Mesothermal vein |
A vein that forms at intermediate depth and temperature |
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Metamorphic rock |
Any rock (e.g. schist, gneiss, etc.) that was formed in some fashion from a pre-existing rock, through heat, pressure, the effect of superheated fluids, or any combination of these forces. |
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Oxidized zone |
The part of an ore body (usually the upper part) that has been altered by downward percolating groundwater, containing dissolved oxygen and carbon dioxide. |
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Pegmatite |
An igneous rock of extremely coarse grain size. Usually found as dikes within a larger plutonic or metamorphic rock mass, pegmatites are often excellent sources of large, fine crystals, especially of quartz, tourmaline, feldspar, and mica. |
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Pipe |
A vertical, cylindrical mass of igneous rock. |
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Placer |
A deposit of heavy mineral particles (e.g. gold) that have weathered out of the bedrock and been concentrated mechanically, usually by the action of streams. |
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Playa |
A desert plain; a shallow basin in which water collects following a rain and is evaporated |
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Plutonic rock |
A granular igneous rock that has solidified at great depth and shows distinct grain texture (e.g. granite, granodiorite). |
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Porphyritic rock |
An igneous rock in which larger crystals (phenocrysts) are enclosed in a fine grained groundmass, which may be crystalline or glassy. |
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Sedimentary rock |
A layered rock, formed through the accumulation and solidification of sediments, which may originally be made up of minerals, rock debris, or animal or vegetable matter. |
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Sill |
A tabular, sheetlike body of intrusive igneous rock, which has been injected between layers of sedimentary or metamorphic rock. |
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Stock |
A small, irregularly shaped body of intrusive igneous rock with a surface area of less than 65 square kilometers. |
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Ultrabasic rock |
Any plutonic igneous rock (e.g. peridotite) with very low silica content (less than that of a basic rock) (see Acidic rock, Basic rock, and Intermediate rock). |
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Vein |
A tabular or sheetlike body of mineral matter (e.g. quartz) cutting across preexisting rock (e.g. granite or gneiss). |