lel
structures which are oriented with reference to crystallographic
directions in the diamond. Striations and well-defined etch figures are
absent.
DUMORTIERITE.
Two
new localities of the hydroborosilicate of aluminum, dumor-tierite,
have been recently found. One is in New Mexico and the other in Nevada.
This mineral, until recently considered very rare, has now been found
in seven States—New York, Colorado, Washington, New Mexico, Nevada,
Arizona, and California, in the last four of which it occurs in
abundance.
The
New Mexico locality is about 100 miles west of El Paso, 12 miles
northwest of Columbus, in the Tres Hermanas Mountains, southern New
Mexico. This-deposit of dumortierite has been referred to in the press
as lapis lazuli. The ledge containing dumortierite is said to be
several feet wide with an outcrop of over 1,000 feet. Many boulders
with seams of the mineral lie on the ground. A sample sent to the
Geological Survey by W. A. Casler, of Deming, N. Mex., shows a quartz
rock, in part sericitized, with narrow seams a quarter of an inch wide
and the same distance apart, composed of spherulites of blue
dumortierite with smaller amounts of colorless spherulites of similar
structure, which are formed of mica. In places the mica spherulites
appear to have been derived from the alteration of the dumortierite; at
other places the two minerals seem to be intergrowths. If large pieces
suitable for cutting and polishing are obtainable, the occurrence
should yield a striking ornamental stone, for the narrow seams of
bright blue dumortierite in the white rock afford a very attractive
contrast. Well-selected and cut pieces, with a narrow band of the
dumortierite, might also yield cut stones of value in jewelry.
Nevada
also has an extensive deposit of dumortierite, and Adolph Knopf, of the
United States Geological Survey, has kindly furnished the following
note on the occurrence:
Dumortierite
occurs in great abundance in the Rochester district, in Humboldt
County, Nev. On the west flank of Lincoln Hill in this district there
is a large area of Triassic rhyolites that are cut by innumerable
veinlets composed of dumortierite and quartz. The rock inclosed between
the veinlets has been highly altered to an aggregate consisting of
andalusite, quartz, dumortierite, and white mica. The dumortierite of
this Nevada occurrence is prevailingly lavender and pink in color, the
blue, usually thought to be characteristic of dumortierite, being very
rare. The veinlets, so far as is now known, range from a friction of an
inch to 6 inches in thickness. The thicker veinlets consist more
largely of quartz than the smaller ones, and some of this quartz is
noteworthy because of its fine rose color. Between this rose-colored
quartz and the pure pink dumortierite all stages of gradation due to
intergrowth can be seen, and under the microscope the clear
rose-colored quartz is found to inclose numerous hair-like fibers of
dumortierite, to which the color of this new variety of rose-colored
quartz is due.
By
careful prospecting of the area in which the dumortierite occurs it is
possible, though by no means certain, that rose-colored quartz of a
quality suitable for polishing as a precious stone may be found.
GARNET.
Small
quantities of garnets are produced each year in several of the States.
In 1917 the mineral of gem quality was obtained from Arizona,
California, Colorado, Idaho, Maine, Montana, New York,
