|
PRECIOUS STONES.
|
173
|
||
|
|
|||
|
electric,
and readily fuses. The specific gravity is 3'29 to 3'30. It is very
brittle, has a hardness of 6-1/2 to 7, and shows several planes of
cleavage. The streak is colourless. It crystallises in thin triclinic
prisms, having thin edges and being somewhat axe shaped (Fig. 38),
hence the name (from Üæéíç). In chemical composition it is a
boro-silicate of aluminium and calcium with iron and manganese, the
latter metals probably accounting for the colour. It is found at St.
Christophe, in Dauphiné, in gneiss, and in the east of Switzerland,
near Dissentis, as well as in several less important localities. It may
readily be distinguished by its colour and appearance in the
dichroscope.
411. Prehnite. .
Prehnite,
although not itself a Zeolite, is in many respects closely allied with
the Zeolites in mode of formation and occurrence. It is found in a
great range of colours, some of them very beautiful. Usually the colour
is some tint of green, but occasionally it passes to a brilliant orange
or a pale pink ; other specimens are colourless. It is mostly
subtransparent to translucent, with a vitreous lustre. It is doubly
refracting, the mean index being 1*626, and shows rather strong
dispersion ; but individual crystals are rare, and usually small, the
material used as a precious stone being a crystalline aggregate. It is
pyro-electric ; the specific gravity is 2'92 to 3.01, and the hardness
is 6 to 6-1/2. The crystalline form is orthorhombic. It usually occurs
in aggregates of minute indistinct crystals strongly coherent, and is
found sometimes in large masses, one in the Heddle collection in the
Royal Scottish Museum measuring 3 feet in length. It is always found as
a product of the
p.s.
ô
|
|||
|
|
|||