Andalusite
Encyclopedia
Andalusite is an aluminium
nesosilicate
mineral
with the chemical formula Al2SiO5.
The variety chiastolite
commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section.
A clear variety first found in Andalusia
, Spain
can be cut into an interesting gemstone. Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence
, although the colors are actually the result of unusually strong pleochroism
.
It is associated with mica schist which increases alkali content in ultimate product and so it has not been exploited economically so far.
mineral which forms under low pressure and moderate to high temperatures. Called Lapis Crucifer in ancient texts. The minerals kyanite
and sillimanite
are polymorphs
of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found.
It was first described and named after the type locality
in the Ronda Massif, Málaga
, Andalusia
, Spain
in 1789.
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
nesosilicate
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
with the chemical formula Al2SiO5.
The variety chiastolite
Chiastolite
The mineral chiastolite is a variety of andalusite with the chemical composition Al2SiO5. It is noted for distinctive cross-shaped black inclusions of graphite. In areas around Georgetown, California, metamorphosed sediments contained andalusite and chiastolite in a graphite rich metasediment...
commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section.
A clear variety first found in Andalusia
Andalusia
Andalusia is the most populous and the second largest in area of the autonomous communities of Spain. The Andalusian autonomous community is officially recognised as a nationality of Spain. The territory is divided into eight provinces: Huelva, Seville, Cádiz, Córdoba, Málaga, Jaén, Granada and...
, Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
can be cut into an interesting gemstone. Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence
Iridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...
, although the colors are actually the result of unusually strong pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
.
It is associated with mica schist which increases alkali content in ultimate product and so it has not been exploited economically so far.
Occurrence
Andalusite is a common regional metamorphicMetamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
mineral which forms under low pressure and moderate to high temperatures. Called Lapis Crucifer in ancient texts. The minerals kyanite
Kyanite
Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...
and sillimanite
Sillimanite
Sillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....
are polymorphs
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found.
It was first described and named after the type locality
Type locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
in the Ronda Massif, Málaga
Málaga
Málaga is a city and a municipality in the Autonomous Community of Andalusia, Spain. With a population of 568,507 in 2010, it is the second most populous city of Andalusia and the sixth largest in Spain. This is the southernmost large city in Europe...
, Andalusia
Andalusia
Andalusia is the most populous and the second largest in area of the autonomous communities of Spain. The Andalusian autonomous community is officially recognised as a nationality of Spain. The territory is divided into eight provinces: Huelva, Seville, Cádiz, Córdoba, Málaga, Jaén, Granada and...
, Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
in 1789.