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A mineraloid is a naturally occurring mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. For example, obsidian is an amorphous glass and not a crystal. Jet is derived from decaying wood under extreme pressure. Opal is another mineraloid because of its non-crystalline nature. Pearl is considered a mineraloid because the included calcite and/or aragonite crystals are bonded by an organic material, and there is no definite proportion of the components.



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Mineral Element or chemical compound that is normally crystalline, formed as a result of geological processes

In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form..

Silicon dioxide chemical compound

Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics (as an electrical insulator), and as components in the food and pharmaceutical industries.

Opal A hydrated amorphous form of silica

Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.

Chert A hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of microcrystalline (or cryptocrystalline) crystals of quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

Volcanic glass material

Volcanic glass is the amorphous (uncrystallized) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas. Volcanic glass can refer to the interstitial, or matrix, material in an aphanitic (fine grained) volcanic rock or can refer to any of several types of vitreous igneous rocks. Most commonly, it refers to obsidian, a rhyolitic glass with high silica (SiO2) content.

Trachyte igneous rock

Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. It is the volcanic equivalent of syenite. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present.. Biotite, clinopyroxene and olivine are common accessory minerals.

Aphanite Igneous rocks which are so fine-grained that their component mineral crystals are not detectable by the unaided eye

Aphanite, or aphanitic as an adjective, is a name given to certain igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye. This geological texture results from rapid cooling in volcanic or hypabyssal environments. As a rule, the texture of these rocks is not the same as that of volcanic glass, with volcanic glass being non-crystalline (amorphous), and having a glass-like appearance.

Volcanic rock Volcanic rocks composing or associated with volcanoes, volcanic activity or volcanism

Volcanic rock is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.

Diatomaceous earth Naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder

Diatomaceous earth, diatomite or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 3 μm to more than 1 mm, but typically 10 to 200 μm. Depending on the granularity, this powder can have an abrasive feel, similar to pumice powder, and has a low density as a result of its high porosity. The typical chemical composition of oven-dried diatomaceous earth is 80–90% silica, with 2–4% alumina and 0.5–2% iron oxide.

Tachylite A form of basaltic volcanic glass

Tachylite is a form of basaltic volcanic glass. This glass is formed naturally by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster. It is very brittle and occurs in dikes, veins and intrusive masses. The word originates from the Ancient Greek takhus meaning swift.

Lechatelierite questionable silica mineral

Lechatelierite is silica glass, amorphous SiO2, non-crystalline mineraloid.


Hyalite is a form of opal with a glassy and clear appearance which may exhibit an internal play of colors if natural inclusions are present. It is also called Muller's glass, water opal and jalite. The name Müller's glass derived from the name of its discoverer, Franz-Joseph Müller von Reichenstein.

Allophane An amorphous to poorly crystalline hydrous aluminium silicate clay mineraloid

Allophane is an amorphous to poorly crystalline hydrous aluminium silicate clay mineraloid. Its chemical formula is Al2O3·(SiO2)1.3-2·(2.5-3)H2O. Since it has short-range atomic order, it is a mineraloid, rather than a mineral, and can be identified by its distinctive infrared spectrum and its X-ray diffraction pattern. It was first described in 1816 in Gräfenthal, Thuringia, Germany. Allophane is a weathering or hydrothermal alteration product of volcanic glass and feldspars and sometimes has a composition similar to kaolinite but generally has a molar ratio of Al:Si = 2. It typically forms under mildly acidic to neutral pH (5–7). Its structure has been debated, but it is similar to clay minerals and is composed of curved alumina octahedral and silica tetrahedral layers. Transmission electron micrographs show that it is generally made up of aggregates of hollow spherules ~3–5 nm in diameter. Allophane can alter to form halloysite under resilicating aqueous conditions and can alter to form gibbsite under desilicating conditions. A copper containing variety cupro-allophane has been reported.

Quartz-porphyry A type of volcanic rock containing large porphyritic crystals of quartz

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.

Hyaloclastite A volcaniclastic accumulation or breccia

Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin margins on the lava flow surfaces and between pillow lavas as well as in thicker deposits, more commonly associated with explosive, volatile-rich eruptions as well as steeper topography. Hyaloclastites form during volcanic eruptions under water, under ice or where subaerial flows reach the sea or other bodies of water. It commonly has the appearance of angular flat fragments sized between a millimeter to few centimeters. The fragmentation occurs by the force of the volcanic explosion, or by thermal shock and spallation during rapid cooling.

In the field of mineralogy, fracture is the texture and shape of a rock's surface formed when a mineral is fractured. Minerals often have a highly distinctive fracture, making it a principal feature used in their identification.

Mineral alteration refers to the various natural processes that alter a mineral's chemical composition or crystallography.

Microlites are minute crystals in an amorphous matrix. In igneous petrology, the term microlitic is used to describe vitric matrix containing microscopic crystals. Microlitic rocks are a type of hypocrystalline rocks. Unlike ordinary phenocrysts, which can be seen with little or no magnification, microlites are generally formed in rapidly cooled (quenched) basaltic lava, where cooling rates are too high to permit formation of larger crystals.


  1. 1 2 3 Peacock, M. A.; Fuller, R. E. (1928). "Chlorophaeite, sideromelane, and palagonite from the Columbia River Plateau" (PDF). American Mineralogist . 13: 360–382. Retrieved 6 September 2017.
  2. Schandl, Eva S.; Gorton, Michael P. (1995). "Phyllosilicate Alteration of Olivine in The Lower Sheeted Dike Complex, Leg 140, Hole 504B" (PDF). Proceedings of the Ocean Drilling Program, Scientific Results. 137/140: 207–216. doi:10.2973/odp.proc.sr.137140.019.1995. ISSN   1096-7451.