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Millerite is a common metamorphic mineral replacing pentlandite within serpentinite ultramafics. It is formed in this way by removal of sulfur from pentlandite or other nickeliferous sulfide minerals during metamorphism or metasomatism. Millerite is also formed from sulfur poor olivine cumulates by nucleation. Millerite is thought to form from ...
The other minerals in the rock are termed accessory minerals, and do not greatly affect the bulk composition of the rock. Rocks can also be composed entirely of non-mineral material; coal is a sedimentary rock composed primarily of organically derived carbon. [34] [38]
Zircon, apatite, titanite, magnetite, ilmenite, and sulfides occur as accessory minerals. [12] Varieties deficient in hornblende and other dark minerals are called leucodiorite. [6] [13] A ferrodiorite is a dioritoid enriched in iron [14] and titanium. Ferrodiorites are common in the lower oceanic crust. [15]
Rutile is a common accessory mineral in high-temperature and high-pressure metamorphic rocks and in igneous rocks. Thermodynamically, rutile is the most stable polymorph of TiO 2 at all temperatures, exhibiting lower total free energy than metastable phases of anatase or brookite. [7]
Zircon is a common accessory to trace mineral constituent of all kinds of igneous rocks, but particularly granite and felsic igneous rocks. Due to its hardness, durability and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands.
Topaz is an accessory mineral to felsic igneous, sedimentary, and hydrothermally altered rocks. [28] The crystal structure of topaz alternates between sheets of (F, OH) 2 O and O along (010) with Al 3+ occupying the octahedral sites and Si 4+ in the tetrahedral sites. [28]
Apatite is very common as an accessory mineral in igneous and metamorphic rocks, where it is the most common phosphate mineral. However, occurrences are usually as small grains which are often visible only in thin section.
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