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Greywacke or graywacke (German grauwacke, signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size lithic fragments set in a compact, clay-fine matrix.
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While pure quartzite is usually white or gray, Baraboo Quartzite is typically dark purple to maroon in color, due to the presence of iron and other impurities. [2] Baraboo Quartzite may display strata created by progressive deposition of layers of sand in the original sandstone from which the quartzite was formed (through metamorphism ). [ 2 ]
Evaporite – Water-soluble mineral deposit formed by evaporation from an aqueous solution; Flint – Cryptocrystalline form of the mineral quartz; Geyserite – Form of opaline silica often found around hot springs and geysers; Greywacke – Sandstone with angular grains in a clay-fine matrix; Gritstone – Hard, coarse-grained, siliceous ...
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A sample of banded hornfels, formed by contact metamorphism of sandstones and shales by a granite intrusion. Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. [1]
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith ) is subjected to temperatures greater than 150 to 200 °C (300 to 400 °F) and, often, elevated pressure of 100 megapascals (1,000 bar ) or more, causing profound physical or chemical changes.