shows
a conchoidal fracture when a fracture at all is obtained, but owing to
a highly perfect cleavage a true fracture is not often seen. This
cleavage is parallel to the faces of the rhombohedron, and advantage is
taken of it in preparing Calcite prisms for the dichroscope and the
polariscope. The streak is white. The crystalline form is rhombohedral,
and the mineral often occurs in a great variety of highly developed
crystals. The crystals are normally attached, and hence not fully
developed.
Its
modes of origin are very various, but the two chief ones are those due
first to the uprising of heated water, and secondly, to the downward
percolation of water usually at moderate temperature. In its hypogene
form it is associated with many metalliferous ores, and in its epigene
form with the great array of minerals formed by decomposition and
subsequent new chemical combination.
The chemical composition is calcium carbonate, CaC03. Some of the calcium is frequently replaced by iron, magnesium, lead, manganese, or zinc.
The
normal crystallised variety only concerns us in so far as it is used
for the optical instruments before mentioned. For this purpose the
large cleavage rhombs from the Icelandic localities on the Eskefjord
and Breitifjord are used, but the material is now becoming scarce.
The
fibrous variety known as Satin Spar is of more special interest; the
fibres are very fine, and are arranged in a parallel manner so as to
exhibit a silky effect when the specimen is broken; some forms show a
wavy band of light also, and the appearance may then be compared to
"watered" silk. The cause of this phenomenon is the same as in other
chatoyant minerals, namely, reflection
