of
limestone. Pyrite in crystals and agglomerations of crystals is
scattered through the rock. This pyrite, along with some
pseudo-morphous limonite, is separated from the rock along with the
sapphires and constitutes the greater part of the concentrates obtained
in washing for the latter. In thin section under the microscope the
biotite is strongly pleochroic, varying from almost colorless to a
strong clear brown color. It occurs abundantly in ragged shreds through
the rock, rarely with crystal form, and contains small apatite
crystals. The pyroxene is pale greenish to colorless and belongs to the
variety diopside. It contains numerous inclusions which may originally
have been glass. No feldspars have been observed in the rock, though a
kaolin-like substance in the interstices may represent a decomposed
mineral, as leucite or analcite, especially since the rock resembles a
known basic analcite basalt.
The
sapphires are scattered through the lamprophyre, none having been
observed associated with the limestone inclusions. One sapphire crystal
was seen embedded in a mass of heavily pyritized lamprophyre. It is
thought the sapphires formed by the crystallization of an excess of
alumina dissolved in the lamprophyre magma. The source of this alumina
is supposed to be in the shales of the several thousand feet of older
formations, underlying the thousand feet of Madison limestone through
which the dike cuts. During the intrusion through the shales fragments
were torn off and floated up with the magma, by which they were
eventually dissolved. When the magma began to solidify the excess of
alumina separated out in the form of sapphire. The sapphires
crystallized out before the magma completely solidified, for some of
them were badly etched and corroded by a partial reabsorption by the
magma after formation. Included fragments of limestone taken in at
higher levels were acted on to varying degrees by the magma. Smaller
masses were metamorphosed to crystallized calcite, and nearly all were
surrounded by diopside rims formed by the reaction on the magma.
The
sapphires range in size from minute crystals up to 4 or 5 carats.
Rarely crystals of 8 or 10 carats are found, the majority, however,
weighing under 3 carats. A large quantity of small sapphire, classed as
"culls," is obtained. This material is in great demand for watch
jewels, for which, through the flattened form of many of the crystals,
it is especially suitable. The color of the Yogo sapphire ranges from a
light blue to the rich characteristic "cornflower" blue of the oriental
sapphire. They make a beautiful gem and are highly prized for their
color and brilliancy. Probably over 90 per cent of the sapphire is of
good blue color and gem quality, the remainder being grayish or of poor
color. Occasional purplish-colored gems are found. The Yogo sapphires
occur in rough crystals whose common forms are the base and a rare
rhombohedral face a;(3032).° The basal planes are roughly striated
parallel to their intersection with the rhombohedron faces. A repeated
development of the base and rhombohedron is not uncommon. The basal
planes are badly etched on some crystals, the etched figures generally
showing a rhombohedral symmetry and several being sometimes grown
together.
New Mine Sapphire Syndicate.—The writer wishes to acknowledge the courtesy shown and the assistance rendered him by Mr. C. T.
" Pratt, J H., Crystallography ol the Montana sapphires: Am. Jour. Sci., 4th ser., vol. 4,1897, pp. 24-428.
51506—m b 190T, pt 2------52
