nearly
equal degree. The indices of a green crystal for yellow light were
found to be 1.719 and 1-718. Dichroism is well marked, a green crystal
showing images in the dichroscope respectively a pure green and a
yellow-green. It fuses before the blowpipe. The specific gravity is 3-35 to 3-45. The fracture is subconchoidal, and the mineral is brittle. The hardness is 6i, and
the streak white. The crystalline form is tetragonal and the crystals
are usually short prisms, terminated by a pyramid and the basal face
(Fig. 30). The crystals are often highly modified. Two cleavages are
present, one a prismatic one and the other parallel to the basal plane.
Idocrase is always found in calcareous rocks which have undergone
thermo-metamorphism (Fig. 31). In chemical composition it is a basic
calcium aluminium silicate, but the exact formula is not very certain,
possibly
Iron, manganeseand
magnesium are sometimes present, and the two first elements may
considerably modify the colour. It is a mineral of wide distribution,
but many forms are opaque or only subtranslucent, and the only
localities from which material has been obtained for gem use are in
Italy, with the exception of the variety Xanthite, a brown Idocrase
found near Amity in New York State. In Italy the brown crystals are
found in the ejected blocks of Monte Somma, of which volcano Vesuvius
is a later cone ; at this locality the mineral is associated with a
great number of interesting species. It also occurs in Piedmont, on the
p.s.
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