Staurolite is a reddish brown to black, mostly opaque,

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 a white streak. It crystallizes in the

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

crystal system, has a

Mohs hardness The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by t ...

of 7 to 7.5 and the chemical formula: Fe²⁺₂Al₉O₆(SiO₄)₄(O,OH)₂.

Magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

and

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

substitute in the iron site and trivalent iron can substitute for aluminium.

Properties

Staurolite often occurs twinned in a characteristic cross-shape, called cruciform penetration twinning. In handsamples, macroscopically visible staurolite

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

s are of prismatic shape. The mineral often forms

porphyroblast A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyrobl ...

s. In

thin section In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron ...

s staurolite is commonly twinned and shows lower first order birefringence similar to

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

, with the twinning displaying optical continuity. It can be identified in

metamorphic rocks 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, caus ...

by its swiss cheese appearance (with poikilitic

) and often mantled porphyroblastic character.

Name

The name is derived from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

, ''stauros'' for cross and ''lithos'' for stone in reference to the common twinning.

Occurrence

Staurolite is a regional metamorphic mineral of intermediate to high grade. It occurs with almandine

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

, micas,

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 ...

; as well as

albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate ...

, biotite, 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 ...

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

and

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 ...

of regional metamorphic rocks. It is the official state mineral of the

U.S. state In the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its sove ...

Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to the ...

and is also to be found in the

Lepontine Alps , topo_map= Swiss Federal Office of Topography swisstopo , photo=MonteLeone0001.jpg , photo_caption=Monte Leone , country_type= Countries , country= , subdivision1_type= Cantons, Regions , subdivision1= , parent= Western Alps , borders ...

in Switzerland. Staurolite is most commonly found in

Fannin County, Georgia Fannin County is a county located in the north central portion of the U.S. state of Georgia. As of the 2020 census, the population was 25,319. It is one of the most rural counties in Georgia due its location in the Appalachian Mountain Range, ...

. It is also found in

Fairy Stone State Park Fairy Stone State Park, located in Patrick County, Virginia, is the largest of the original six state parks that opened on June 15, 1936, and is home to the mysterious "fairy stones", or staurolite. The stone, prevalent in the region, may have th ...

Patrick County, Virginia Patrick County is a county located on the central southern border of the Commonwealth of Virginia. As of the 2020 census, the population was 17,608. Its county seat is Stuart. It is located within both the rolling hills and valleys of the Pi ...

. The park is named for a local name for staurolite from a legend in the area.Virginia State Parks
/ref> Samples are also found in Island Park, Idaho, near

Henrys Lake Henrys Lake is a small, shallow alpine lake in the western United States, in eastern Idaho. Approximately in area, at in length and in width, its surface elevation is above sea level. It is on the southwest side of the Henrys Lake Mountains ...

;

Taos Taos or TAOS may refer to: Places * Taos, Missouri, a city in Cole County, Missouri, United States * Taos County, New Mexico, United States ** Taos, New Mexico, a city, the county seat of Taos County, New Mexico *** Taos art colony, an art colo ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ke ...

; near Blanchard Dam in

Minnesota Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over to ...

; and

Selbu Selbu is a municipality in Trøndelag county, Norway. The administrative centre of the municipality is the village of Mebonden. Other villages in Selbu include Flora, Fossan, Hyttbakken, Innbygda, Selbustrand, Trøa, Tømra, and Vikvarvet. T ...

, Norway.

Use

Staurolite is one of the

index mineral An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith (parent rock), chemical reactions between mine ...

s that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.

References

Mineral galleries
{{Authority control Magnesium minerals Iron(II) minerals Zinc minerals Aluminium minerals Nesosilicates Symbols of Georgia (U.S. state) Monoclinic minerals Minerals in space group 12