Malachite
is a green carbonate of copper containing water, the percentages being
in the typical mineral: cupric oxide 71.9, carbon dioxide 19.9, and
water 8.2. It is the common form which copper assumes when it, or even
its ores, oxidize in the air. Many of the green stains on rocks, or
minerals, can be correctly referred to malachite. It is only valued for
ornamental purposes, however, when it occurs in compact masses, usually
exhibiting concentric layers. Malachite in this form takes a fine
polish. Malachite is not a hard mineral, its hardness being between 3.5
and 4. It can, therefore, be scratched with a knife. It is
comparatively heavy, weighing four times as much as an equal bulk of
water. When heated before the blowpipe it fuses easily, coloring the
flame green. By heating long enough on charcoal it can be made to yield
a globule of copper. It is easily attacked by common acids, causing
effervescence of carbon dioxide. This test can be used to distinguish
it from the silicate of copper, chryscolla, which has the same color.
Besides
its occurrence in massive forms, as noted above, malachite not
uncommonly is found in tufts and rosettes incrusting other minerals.
This is an especially common occurrence in mines in Arizona, and
affords specimens of great beauty, especially when the green tufts of
malachite are seen upon brown limonite, for then the appearance of moss
on wood is closely simulated. Such material is, of course, too fragile
to be used for decorative purposes.
Malachite
is prepared for ornamental use by sawing masses of the character of
those previously referred to into thin strips, which are then fastened
as a veneer on vessels of copper, slate, or other stone previously
turned to the desired shape. Putting pieces together so that neither by
their outlines nor color will it appear that they are patchwork,
requires a high degree of skill, and such work is done almost
exclusively in Russia. Table tops, vases, and various other vessels are
manufactured in this way, and form objects of great beauty. The pillars
of the Church of Isaac, in St. Petersburg, are of malachite prepared in
this way, and there are similar pillars in the Church of St. Sophia,
Constantinople, said to have been taken from the Temple of Diana at
Ephesus.
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