Spurrite

Spurrite
Spurrite from New Mexico
General
Category	Nesosilicate
Formula (repeating unit)	Ca₅(SiO₄)₂CO₃
Strunz classification	9.AH.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P2₁/a
Identification
Color	Gray, gray white to lilac gray
Crystal habit	Massive to granular
Twinning	Polysynthetic twins on [001] and [101]
Cleavage	Distinct on [100]
Fracture	Uneven to splintery
Mohs scale hardness	5
Luster	vitreous to resinous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3
Optical properties	Biaxial (-)
Refractive index	nα = 1.640–1.641 nβ = 1.674–1.676 nγ = 1.679–1.681
Birefringence	δ = 0.039–0.040
Other characteristics	Green cathodoluminescence
References

Spurrite is a white, yellow or light blue mineral with monoclinic crystals. Its chemical formula is Ca₅(Si O₄)₂CO₃.

Spurrite is generally formed in contact metamorphism zones as mafic magmas are intruded into carbonate rocks. Spurrite's space group is P 2/a. It is biaxial with a birefringence of 0.0390–0.0400, giving second order red interference colors when viewed under crossed polarizers in a petrographic microscope.

The calcium is in six-fold coordination with the oxygen, the silicon is in a four-fold coordination with the oxygen and the carbon is in two-fold coordination. One unique characteristic of spurrite is that it actually abides by two twin laws. Polysynthetic twinning can occur along its (001) and another type of twinning can occur parallel to its optical axes.

Spurrite was first described in 1908 for an occurrence in the Terneras Mine, Velardeña District, Durango, Mexico. It was named for American economic geologist Josiah Edward Spurr (1870–1950).

In addition to its type locality, spurrite has been reported from Riverside County, California; Luna County, New Mexico; and from the Little Belt Mountains, Lewis and Clark County, Montana. It is also found in Ireland, Scotland, New Zealand, Turkey, Israel, Japan and Siberia.

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