EUCLASE 



325 



have a perfect cleavage parallel to their one plane of symmetry ; this cleavage plane 

 truncates the two acute edges of the rhombic prism. Owing to the perfect cleavage, 

 crystals of euclase are liable to become fissured and then broken, and when being cut, unless 

 exceptional care is taken, they chip at the edges. 



The mineral has a hardness (H = 7|) slightly exceeding that of beryl ; it is somewhat 

 heavy, having a specific gravity of 3'05 to .'5-10 ; when rubbed it acquires a not inconsiderable 

 charge of electricity. Its lustre is vitreous, but in the direction of 

 cleavage is sometimes pearly ; the mineral is susceptible of a high 

 polish and is frequently perfectly clear and transparent. Its refrac- 

 tion, double refraction, and dispersion are all small. Its colour 

 resembles in many respects that of precious beryl, being either green 

 with a tinge of blue (Plate XIII., Fig. 5), or green with a yellowish 

 tinge; it is almost invariably pale in shade, deeply coloured stones 

 being rare and perfectly colourless ones quite unusual. Stones of a 

 rather deep blue-green colour, as represented in the plate just quoted, 

 are most admired ; they resemble some aquamarines and blue topaz 

 very closely, but may be easily distinguished from either of these by 

 the difference in the specific gravity and by the existence of a distinct 

 dichroism in euclase. 



Very few localities for euclase are known, and nowhere is it found in abundance. It 

 was first met with in Brazil, in the neighbourhood of Ouro Preto (formerly known as 

 Villa liica), in Minas Geraes. It occurs here, associated with yellow topaz, in nests in the 

 quartz veins by which the clay-slates, which accompany itacolumite, are traversed. It would 

 appear, however, that topaz and euclase are never found acbually side by side in the same 

 nest or druse. The principal locality is Boa Vista, near Ouro Preto (Fig. 67). A euclase 

 weighing over li pounds was reported by I^. von Eschwege from this district, but the 

 majority of the stones found thei-e are much smaller, and, moreover, they are often broken 

 into fragments along the cleavage plane. 



Euclase is also found in the gold-washings of the Sanarka river in the Ural Mountains, 

 situated in the Government of Orenburg. It occurs here in loose crystals, many of which, 

 as in Brazil, are merely cleavage fragments ; they may reach a length of 1 J inches, but, as 

 a rule, are much smaller. They vary in colour from grass-green to greenish-blue, and are 

 associated with topaz, chrysoberyl, and other minerals. 



Small crystals of euclase, of a pale yellowish colour, have been found in recent years in 

 mica-schist in the Grossglockner district of the Austrian Alps ; this occurrence, however, is 

 solely of mineralogical interest. 



Fig. 64. Crystalline 

 I'orm of euclase. 



