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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.[3]

Euclase
Euclase-denv08-26a.jpg
General
CategoryNesosilicate
Formula
(repeating unit)
BeAlSiO4(OH)
Strunz classification9.AE.10
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/a
Unit cella = 4.763, b = 14.29
c = 4.618 [Å]; β = 100.25°; Z = 4
Identification
ColorColorless, white, pale green to deep yellowish green, greenish blue, pale blue to deep blue
Crystal habitPrismatic crystals
CleavagePerfect, perfect on {010}, imperfect on {110} {001}
FractureConchoidal
TenacityBrittle
Mohs scale hardness7.5
LusterVitreous
StreakWhite
DiaphaneityTransparent, translucent
Specific gravity2.99 - 3.1
Optical propertiesBiaxial (+)
Refractive indexnα = 1.652 nβ = 1.655 nγ = 1.671
Birefringenceδ = 0.019
PleochroismMay be marked in shades of deep blue
2V angle50°
Dispersionr > v
References[1][2][3]

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).[4]

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,[2] where it occurs with topaz. It is found rarely in the mica-schist of the Rauris in the Austrian Alps.

ReferencesEdit

  1. ^ Euclase data on Webmineral
  2. ^ a b Euclase on Mindat.org with location data
  3. ^ a b Euclase in the Handbook of Mineralogy, Mineral Data Publishers PDF
  4. ^ Chisholm, Hugh, ed. (1911). "Euclase" . Encyclopædia Britannica (11th ed.). Cambridge University Press.