feet
above sea, and one-half of a mile south of the top of Mount Antero, on
the west slope of Mount Antero, and on smaller peaks to the east of it.
Former glaciers have left their marks in the region by cirques,
moraines, and lakelets in the surrounding valleys. The gem locality is
reached from the little town of Alpine, or from Fisher station, on the
narrow-gage branch of the Colorado and Southern Railroad between Buena
vista and Gunnison. From Alpine the distance is about 8 miles and the
climb about 5,000 feet. Over half of the trip can be made on burro or
mule back.
The
mass of Mount Antero and White Mountain is composed of quartz monzonite
with local variations. Along the divide half a mile southwest of White
Mountain an abrupt change of character to more basic rock is indicated
by an area of dark-brown soil on Calico Mountain. The talus slopes and
angular gravel-covered surfaces of Mount Antero and White Mountain
appear light gray and frost-white in contrast to Calico Mountain. On
parts of White Mountain, especially near the beryl deposits, the rock
is muscovite granite composed of orthoclase with microperthite
intergrowths, oligoclase, quartz, muscovite, and a little biotite.
Magnetite, apatite, and zircon are present as accessories.
The
beryl, quartz, phenacite, and topaz crystals occur in micro-litic
cavities and pockets in the granite and granitic phases of the quartz
monzonite. The pockets occur in streaks or are isolated and occupy no
definite arrangement with respect to each other. The crystallization
along the streaks and around the miarolitic cavities is coarse, and the
rock may be called pegmatite. One streak-or vein of pegmatite with its
gem-bearing pockets is 15 inches thick where exposed in a pit. This
streak can t>e traced for some distance in a northeast direction
across the ridge near the top of White Mountain by loose beryl and
quartz crystals in the angular gravel soil formed by the disintegration
of the rock and in blocks of the granite lying on the surface. As
exposed in the pit the vein consists of pegmatite, much crystallized
clear and smoky quartz penetrated by beryl crystals, crystals of
phenacite, muscovite mica, feldspar, violet and green-colored fluorite,
etc. Crystals of beryl and quartz occur scattered along the top of the
ridge from the summit of White Mountain for about 200 yards to the
southwest. In places many crystals may be found loose in the gravel,
indicating the position or a gem pocket.
With
the exception of smoky and clear quartz, beryl is the most abundant
specimen and gem mineral found at the Mount Antero locality. The beryl
occurs in transparent clear crystals from small size to those measuring
2 centimeters in diameter and 6 centimeters in length. Many of the
crystals are badly etched or corroded, especially on the ends
projecting into cavities. The color of the beryls varies from clear
light blue to pale and deep aquamarine green. Blue beryl is very
plentiful "frozen" in the rock, either in the granite, pegmatite, or
quartz. Much of the frozen beryl is badly checked and flawed. That from
the cavities is generally clear and of gem quality. The other
minerals—phenacite, topaz, and fluorite—are closely associated with the
quartz and beryl, and often attached to them. Phenacite crystals attain
the size of the beryl and are generally colorless. S. L. Penfielda has described the beryl, bertrandite, and phenacite from Mount Antero. He discussed the etching of the
a Beryllium minerals from Colorado: Am. Jour. Scl., 3d Ser., vol. 40, 1890, pp 489-491.
