Euclase

Euclase
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General
Category	Nesosilicate
Formula (repeating unit)	BeAlSiO4(OH)
Strunz classification	09.AE.10
Crystal symmetry	Monoclinic prismatic H-M symbol: (2/m) Space group: P 21/a
Unit cell	a = 4.763 Å, b = 14.29 Å, c = 4.618 Å, β = 100.25°; Z=4
Identification
Color	Colorless, white, pale green to deep yellowish green, greenish blue, pale blue to deep blue
Crystal habit	Prismatic crystals
Crystal system	Monoclinic
Cleavage	Perfect, perfect on {010}, imperfect on {110} {001}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	7.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent, translucent
Specific gravity	2.99 - 3.1
Optical properties	Biaxial (+)
Refractive index	nα = 1.652 nβ = 1.655 nγ = 1.671
Birefringence	δ = 0.019
Pleochroism	May be marked in shades of deep blue
2V angle	50°
Dispersion	r > v
References

Euclase is a beryllium aluminium hydroxide silicate mineral (BeAlSiO₄(OH)). It crystallizes in the monoclinic crystal system and is typically massive to fibrous as well as in slender prismatic crystals. It is related to beryl (Be₃Al₂Si₆O₁₈) and other beryllium minerals. It is a product of the decomposition of beryl in pegmatites.

Euclase crystals are noted for their blue color, ranging from very pale to dark blue. The mineral may also be colorless, white, or light green. Cleavage is perfect, parallel to the clinopinacoid, and this suggested to René Just Haüy the name euclase, from the Greek εὖ, easily, and κλάσις, fracture. The ready cleavage renders the crystals fragile with a tendency to chip, and thus detracts from its use for personal ornament. When cut it resembles certain kinds of beryl and topaz, from which it may be distinguished by its specific gravity (3.1). Its hardness (7.5) is similar to beryl (7.5 - 8), and a bit less than that of topaz (8).

Euclase, 3.0 x 1.6 x 1.6 cm. Lost Hope Mine, Mwami, Mashonaland West Province, Zimbabwe

It was first reported in 1792 from the Orenburg district in the southern Urals, Russia, where it is found with topaz and chrysoberyl in the gold-bearing gravels of the Sanarka (nowadays probably, Sakmara River, Mednogorsk district, Orenburgskaya Oblast'). Its type locality is Ouro Prêto, Minas Gerais, Southeast Region, Brazil, where it occurs with topaz. It is found rarely in the mica-schist of the Rauris in the Austrian Alps.