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Datolite on sphalerite from San Luis Potosí, Mexico (size: 6.2 x 5.3 x 2.8 cm)
Category Nesosilicate
(repeating unit)
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
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 [1][2][3]

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 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.


  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 domainChisholm, Hugh, ed. (1911). "Datolite". Encyclopædia Britannica. 7 (11th ed.). Cambridge University Press. p. 846. 

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