Celestite

Celestine
Clear grey-blue celestine crystal crust from Madagascar
General
Category	Sulfate minerals
Formula (repeating unit)	SrSO4 sometimes contains minor calcium and/or barium
Strunz classification	7.AD.35
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Unit cell	a = 8.359 Å, b = 5.352 Å, c = 6.866 Å; Z = 4
Identification
Color	Colorless, white, pale blue, pink, pale green, pale brown, black
Crystal habit	Tabular to pyramidal crystals, also fibrous, lamellar, earthy, massive granular
Cleavage	Perfect on {001}, good on {210}, poor on {010}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	3 - 3.5
Luster	Vitreous, pearly on cleavages
Streak	white
Diaphaneity	Transparent to translucent
Specific gravity	3.95 - 3.97
Optical properties	Biaxial (+)
Refractive index	nα = 1.619 - 1.622 nβ = 1.622 - 1.624 nγ = 1.630 - 1.632
Birefringence	δ = 0.011
Pleochroism	Weak
2V angle	Measured: 50° to 51°
Dispersion	Moderate r < v
Ultraviolet fluorescence	Short UV=yellow, white blue, long UV=yellow, white blue
References

Celestine or celestite (SrSO₄) is a mineral consisting of strontium sulfate. The mineral is named for its occasional delicate blue color. Celestine is the principal source of the element strontium, commonly used in fireworks and in various metal alloys.

Celestine occurs as crystals, and also in compact massive and fibrous forms. It is mostly found in sedimentary rocks, often associated with the minerals gypsum, anhydrite, and halite.

The mineral is found worldwide, usually in small quantities. Pale blue crystal specimens are found in Madagascar.

The skeletons of the protozoan Acantharea are made of celestine, unlike those of other radiolarians which are made of silica.

In carbonate marine sediments, burial dissolution is a recognised mechanism of celestine precipitation.

It is sometimes used as a gemstone.

Celestine crystals are found in some geodes. The world's largest known geode, a celestine geode 35 feet (10.7 m) in diameter at its widest point, is located near the village of Put-in-Bay, Ohio, on South Bass Island in Lake Erie. The geode has been converted into a viewing cave, Crystal Cave, with the crystals which once composed the floor of the geode removed. The geode has celestine crystals as wide as 18 inches (46 cm) across, estimated to weigh up to 300 pounds (135 kg) each.