Staurolite is a reddish brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic 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 ...

of 7 to 7.5 and the chemical formula: Fe²⁺₂Al₉O₆(SiO₄)₄(O,OH)₂. Magnesium, zinc and manganese 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 crystals are of prismatic shape. The mineral often forms porphyroblasts. 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 Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringe ...

similar to quartz, 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, causi ...

by its swiss cheese appearance (with poikilitic quartz) and often mantled porphyroblastic character.

Name

The name is derived from the Greek, ''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, 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 generall ...

; 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 Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumi ...

, and sillimanite in gneiss and schist 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 sov ...

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 in Switzerland. Staurolite is most commonly found in

Fannin County, Georgia Fannin County is a County (United States), county located in the north central portion of the U.S. state of Georgia (U.S. state), Georgia. As of the 2020 United States Census, 2020 census, the population was 25,319. It is one of the most rural c ...

. 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 ...

. 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 Island Park is a city in Fremont County, Idaho, United States. The city's population was 286 at the 2010 census, up from 215 in 2000. The city was incorporated by owners of the many lodges and resorts along U.S. Route 20 in 1947, primarily to c ...

, 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 Mountai ...

;

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; near Blanchard Dam in Minnesota; 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. ...

, 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