staurolite

n.
A brownish to black mineral, chiefly (FeMg)₂Al₉Si₄O₂₃(OH), often having crossed intergrown crystals and sometimes used as a gem.

[Greek stauros, cross + -LITE.]

staurolitic stau'ro·lit'ic (-lĭt'ĭk) adj.

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A nesosilicate mineral occurring in metamorphic rocks. The chemical formula of staurolite may be written as A₄B₄C₁₈D₄T₈O₄₀X₈, where A = Fe²⁺, Mg; B = Fe²⁺, Zn, Co, Mg, Li, Al, Fe³⁺, Mn²⁺; C = Al, Fe³⁺, Cr, V, Ti; D = Al, Mg; T = Si, Al; X = OH, F, O. Staurolite occurs as well-formed, often-twinned, prismatic crystals. It is brown-black, reddish brown, or light brown in color and has a vitreous to dull luster. Light color and dull luster can result from abundant quartz inclusions. There is no cleavage, specific gravity is 3.65–3.75, and hardness is 7–7.5 (Mohs scale). See also Hardness scales.

Typical minerals occurring with staurolite are quartz, micas (muscovite and biotite), garnet (almandine), tourmaline, and kyanite, sillimanite, or andalusite. Staurolite is common where pelitic schists reach medium-grade metamorphism. Examples are the Swiss and Italian Alps (notable at Saint Gotthard, Switzerland), and all the New England states, Virginia, the Carolinas, Georgia, New Mexico, Nevada, and Idaho. See also Metamorphism; Silicate minerals.

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FeAl
Monoclinic -- prismatic (pseudo-orthorhombic)

Environment

Embedded "pressure" mineral which, like almandine garnet, grows as regional metamorphism forms schist and gneiss.

Crystal description

Always crystallized, commonly with two individuals intergrown at right angles (twinned) so as to produce a cross; they may also intergrow at other angles. To 2 in. (5 cm) in length.

Physical properties

Dark brown. Luster glassy; hardness 7-7Ɖ; specific gravity 3.6-3.7; fracture subconchoidal; cleavage fair pinacoidal. Translucent to almost transparent.

Composition

Iron aluminum silicate; can be regarded chemically as a mixture of kyanite with iron hydroxide (15.8% FeO, 55.9% Al ₂ O ₃ , 26.3% SiO ₂ , 2.0% H ₂ O).

Tests

Infusible and insoluble, but after flame roasting, grain crushes easily to a brown, weakly magnetic powder.

Distinguishing characteristics

Since staurolite is always in schist in typical brown crystals, with some individuals commonly twinned, it presents no special recognition problem. Andalusite appears to be unaffected by the blowpiping; dravite tourmaline of this color would fuse.

Occurrence

Large and well-formed crystals are found in Fannin and Cherokee counties, Georgia, and near Taos, New Mexico. Smaller ones are commercially exploited in Fairfax Co., Virginia, as good luck charms. However, many of the twins one sees appear to have been carved from a soft clay, probably dyed brown, and are not the real mineral, though some may be pseudomorphs.

Untwinned, lustrous crystals of more than usual translucency are associated with the blue kyanite of Pizzo Forno, Switzerland, and make attractive specimens, lying in their matrix of fine-grained white mica. Still larger ones, likewise more often untwinned, are found in Bahia and in Minas Gerais, Brazil.

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