Datolite

Datolite
Datolite on sphalerite from San Luis Potosí, Mexico (size: 6.2 x 5.3 x 2.8 cm)
General
Category	Nesosilicate
Formula (repeating unit)	CaBSiO4(OH)
Strunz classification	9.AJ.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 9.62, b = 7.6 c = 4.84 [Å]; β = 90.15°; Z = 4
Identification
Color	Colorless or white; may be grayish, yellow, green, red, pink
Crystal habit	Crystal prismatic, short to tabular; Botryoidal or globular with columnar structure; granular to compact; cryptocrystalline
Cleavage	None
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	5 to 5.5
Luster	Vitreous, rarely subresinous on fracture surface
Diaphaneity	Transparent to translucent, rarely opaque
Specific gravity	2.96 – 3.00
Optical properties	Biaxial (-)
Refractive index	nα = 1.626 nβ = 1.653 - 1.654 nγ = 1.670
Birefringence	δ = 0.044
2V angle	Measured: 74°
Dispersion	r > v; weak
Ultraviolet fluorescence	Fluoresces blue under SW UV
References

Datolite is a calcium boron hydroxide nesosilicate, CaBSiO₄(OH). It was first observed by Jens Esmark in 1806, and named by him from δατεῖσθαι, "to divide," and λίθος, "stone," in allusion to the granular structure of the massive mineral.

Datolite crystallizes in the monoclinic system forming prismatic crystals and nodular masses. The luster is vitreous and may be brown, yellow, light green or colorless. The Mohs hardness is 5.5 and the specific gravity is 2.8 - 3.0.

The type localities are in the diabases of the Connecticut River valley and Arendal, Aust-Agder, Norway. Associated minerals include prehnite, danburite, babingtonite, epidote, native copper, calcite, quartz and zeolites. It is common in the copper deposits of the Lake Superior region of Michigan. It occurs as a secondary mineral in mafic igneous rocks often filling vesicles along with zeolites in basalt. Unlike most localities throughout the world, the occurrence of datolite in the Lake Superior region is usually fine grained in texture and possesses colored banding. Much of the coloration is due to the inclusion of copper or associated minerals in progressive stages of hydrothermal precipitation.