cubical
crystals of it are found, especially where the grains have not traveled
far from the mother rock and so have not become worn or beaten out of
their original shape. All native platinum is more or less magnetic. The
degree of magnetism which it possesses seems to depend directly upon
the percentage of iron with which it is alloyed; thus, when the iron
content is extraordinarily high, say 18 per cent, the crude platinum
may be so magnetic that after treatment with a magnet the fine grains
will clot together, each becoming a miniature lodestone. On the other
hand, when the percentage of iron is low, say 4 per cent, only a
powerful electro-magnet will reveal the fact that the platinum has any
magnetic properties.
Crude
platinum is not essentially affected by hydrochloric, nitric, or
sulphuric acid, but it dissolves in aqua regia and is attacked by free
chlorine and bromine. It does not amalgamate with quicksilver, and when
cold it is not affected by potassium cyanide. It is fusible with great
difficulty, pure platinum melting at 1,750° C.
Associated metals.—The
other minerals of the platinum group sometimes found associated with
crude platinum are platiniridium, palladium, and iridosmium.
Platiniridium
is an alloy of platinum and iridium, in which the iridium attains a
proportion as high as 80 per cent. The physical appearance of the
mineral closely resembles that of crude platinum, but its specific
gravity (22.6 to 23) and its hardness (6.7) are much in excess of
those of the latter metal.
Palladium
is extremely rare. It occurs both in octahedral crystals and in small
grains and scales. It is steel gray in color, and has a metallic
luster. Its specific gravity is 11.3 to 11.8, its hardness is 4.5 to 5,
and it is malleable, sectile, and ductile.
Iridosmium,
the alloy of iridium and osmium already referred to, unlike platinum,
generally contains small percentages of ruthenium. It crystallizes in
the hexagonal system, but usually occurs in irregularly shaped grains
and thin scales. Its hardness is 6.7 and its specific gravity is 18.8
to 21.12. It has a metallic luster and ranges in color from tin white
to steel gray. The quantity of platinum contained in the mineral is
generally under 3 per cent, and the iron content varies from a trace to
1.5 per cent.
The
following table gives the results of a magnetic test made by the writer
at Portland, Oreg., in 1905, upon a small sample of crude platinum
which contained a large percentage of iridosmium. The sample came from
a placer of the Waldo district, southern Oregon. The machine used was a
regulation type of the modern Wetherell magnetic concentrator.
Magnetic test of crude platinum containing iridosmium, 1905.
