270 PRECIOUS STONES.
unknown.
It is found in good specimens in Brazil near Villa Rica; in the Alps on
Monte Campione in the St. Gothard district in schist; in the Tyrol; on
Mount Greiner in the Zillerthal; and near Baskerville, in North
Carolina, in the United States. The colours often resemble Sapphire,
but Cyanite is softer and less dense.
403. Euclase.
Euclase
is a rare mineral, occurring in crystals of a green or greenish blue
colour, often transparent, and having a vitreous lustre. It is doubly
refracting, though only in a feeble degree, the greatest and least
indices for yellow light being 1'671 and 1"652. The dispersion is also
small, but there is a distinct pleochroism. It is easily electrified by
friction, and on strongly heating gives off water. The specific gravity
is 3'05 to 3"10. It is brittle, shows a con-choidal fracture, and has
three cleavages parallel to the three primary crystal forms. It occurs
in short monosym-metric prisms vertically striated, the crystals often
being highly modified.
In composition it is a hydrated beryllium aluminium silicate, H20, 2 BeO, A1203, 2 Si02.
It
occurs in chlorite schist near Novas Minas, in Brazil, with yellow
Topaz; also near the Sanarka River, in the Urals, in gold-bearing
alluvium, with Topaz and Corundum. It has also been found in very small
crystals in the Alps.
It
maybe distinguished from Beryl (variety Aquamarine) by being of higher
specific gravity, and more strongly dichroic, and from blue Topaz by
being less dense and more dichroic.
