|
cleavages
of the mineral is not quite a right angle. The common color of
microcline is white to pale yellow, but occasionally green and red
occur.
It
is only to the green variety that the name of amazonstone is applied, a
name meaning stone from the Amazon River. It first referred probably to
jade, or some such green stone from that locality, and then came to
include green feldspar. No occurrence of green feldspar in that region
is now known.
Practically
all the amazonstone now used for ornamental purposes comes from three
localities. These are the vicinity of Miask in the Ural Mountains,
Pike's Peak, Colorado, and Amelia Court House, Virginia. In all these
places the amazonstone occurs in coarse-grained granite, and is
accompanied by quartz and mica. All gradations are found in color from
the deep green to white, only the bright green being prized for
ornamental purposes. The feldspar is usually well crystallized, and
crystals of several pounds' weight may be found. A crystal will rarely
be of a uniform color, streaks of paler green or white being commonly
present. Only the uniformly colored portions are prized for ornamental
purposes. The green often takes on a bluish tone, and blue sometimes
even predominates. The color is doubtless due to some organic matter,
as it disappears on heating, leaving the stone white. The stone is
always opaque. Its use is not extensive, its sale being chiefly to
tourists in the vicinity of the regions where it is found. Several
other localities in the United States besides those mentioned afford
the mineral, though not in large quantities. It occurs in two or three
localities in North Carolina ; in Paris, Maine; Mount Desert, Maine;
Rockport, Massachusetts; and Delaware County, Pennsylvania. The finest
comes from the Pike's Peak locality. Kunz states that when crystals
from the latter locality were first exhibited at the Centennial
Exposition in Philadelphia, in 1876, they were a great surprise to
Russian dealers, who had brought over some amazonstone from the Urals,
expecting to sell it at what would now be considered fabulously high
prices.
The
second species of feldspar which may be mentioned as of use as an
ornamental stone is labradorite. This differs in composition from
amazonstone in containing soda and lime in place of potash, the
percentages in a typical labradorite being, silica 53.7, alumina 29.6,
lime 11.8, and soda 4.8. Labradorite has the typical cleavage of
feldspar and cleavage surfaces in the direction of easiest cleavage are
usually marked by rows of parallel striae. These show that the mass is
made up of a series of crystal twins in parallel position, and afford
an excellent criterion for determining a triclinic feldspar.
Labradorite is a common rock-forming mineral, especially in the older
rocks. It is only, how-
176
|
||
|
|
||