Datolite

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.[4]

Datolite
Datolite on sphalerite from San Luis Potosí, Mexico (size: 6.2 x 5.3 x 2.8 cm)
General
CategoryNesosilicate
Formula
(repeating unit)		CaBSiO4(OH)
Strunz classification9.AJ.20
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/c
Unit cella = 9.62, b = 7.6
c = 4.84 [Å]; β = 90.15°; Z = 4
Identification
ColorColorless or white; may be grayish, yellow, green, pale green, red, pink, etc.
Crystal habitCrystal prismatic, short to tabular; Botryoidal or globular with columnar structure; granular to compact; cryptocrystalline
CleavageNone
FractureConchoidal to uneven
TenacityBrittle
Mohs scale hardness5 to 5.5
LusterVitreous, rarely subresinous on fracture surface
DiaphaneityTransparent to translucent, rarely opaque
Specific gravity2.96 – 3.00
Optical propertiesBiaxial (-)
Refractive indexnα = 1.626 nβ = 1.653 - 1.654 nγ = 1.670
Birefringenceδ = 0.044
2V angleMeasured: 74°
Dispersionr > v; weak
Ultraviolet fluorescenceFluoresces blue under SW UV
References[1][2][3]

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.

Polished datolite nodule from the Quincy Mine of Michigan’s Copper Country (size: 4.1 x 3.3 x 1.7 cm)

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.

Botryolite is a botryoidal form of datolite.

References
  1. Handbook of Mineralogy
  2. Mindat
  3. Webmineral
  4.  One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Datolite". Encyclopædia Britannica. 7 (11th ed.). Cambridge University Press. p. 846.
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