Andalusite is an
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
nesosilicate
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, silica (silicon dioxide, ) is usually consid ...
mineral
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.John P. Rafferty, ed. (2 ...
with the chemical formula Al
2SiO
5. This mineral was called andalousite by Delamétehrie, who thought it came from
Andalusia
Andalusia (, ; es, Andalucía ) is the southernmost autonomous community in Peninsular Spain. It is the most populous and the second-largest autonomous community in the country. It is officially recognised as a "historical nationality". The ...
, Spain. It soon became clear that it was a locality error, and that the specimens studied were actually from El Cardoso de la Sierra, in the Spanish province of
Guadalajara
Guadalajara ( , ) is a metropolis in western Mexico and the capital of the state of Jalisco. According to the 2020 census, the city has a population of 1,385,629 people, making it the 7th largest city by population in Mexico, while the Guadalaj ...
, not Andalusia.
Andalusite is trimorphic with
kyanite
Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks gener ...
and
sillimanite
Sillimanite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, Connecticut.
Occurrence ...
, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an
aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.
Varieties
The variety
chiastolite commonly contains dark inclusions of carbon or clay which form a
cruciform
Cruciform is a term for physical manifestations resembling a common cross or Christian cross. The label can be extended to architectural shapes, biology, art, and design.
Cruciform architectural plan
Christian churches are commonly describe ...
pattern when shown in cross-section. This stone was known at least from the sixteenth century, being taken to many European countries, as a souvenir, by pilgrims returning from Santiago de Compostela.
Viridine is a
green
Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a combin ...
variety of andalusite in which manganese substitutes for aluminium. Kanonaite is a greenish-black mineral related to andalusite and having the approximate composition
A clear variety found in Brazil and Sri-Lanka can be cut into a gemstone. Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of
iridescence
Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
, although the colors are actually the result of unusually strong
pleochroism.
Occurrence
Andalusite is a common
metamorphic
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 and, often, elevated pressure of or more, causi ...
mineral which forms under low pressure and low to high temperatures. The minerals
kyanite
Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks gener ...
and
sillimanite
Sillimanite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, Connecticut.
Occurrence ...
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 is particularly associated with
pelitic
A pelite ( Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary roc ...
metamorphic rocks such as
mica schist
Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...
.
Uses
Andalusite is used as a
refractory
In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
in furnaces, kilns and other industrial processes.
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...
possesses by far the largest portion of the world's known andalusite deposits.
See also
*
List of minerals
This is a list of minerals for which there are articles on Wikipedia.
Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various ''species''. Within a m ...
References
{{Authority control
Aluminium minerals
Nesosilicates
Orthorhombic minerals
Minerals in space group 58
Industrial minerals