K-feldspar (orthoclase, microcline,
sanidine and related varieties)
Microcline in a Granite
These photos show a large grain of microcline in a granite from near Barre, Vermont. Microcline is characterized by a cross-hatched twinning -- here very well developed -- which is visible only in the XP view. Microcline twinning is sometimes called "tartan plaid" or "Scotch plaid" twinning. Also present in this view are quartz (clear, PP; white to gray, XP) and two flakes of dark brown biotite.
The field of view is 2.5 mm.
Microcline with Quartz, Hornblende and Biotite in an Intermediate Gneiss
These photos show several grains of microcline (K-feldspar) with well developed cross-hatched twinning (XP). Quartz and plagioclase are also present; they have color and interference colors similar to microcline's but lack microcline twinning. Brown biotite and green hornblende can be seen in the PP view. One hornblende grain has a diamond shape and shows hornblende's characteristic two cleavages at angles of 60o and 120o. A few grains of magnetite are in the lower right.
The sample comes from near Flin Flon, Manitoba.
The field of view is 2.5 mm.
Microcline and Hedenbergite in a Syenite from Quebec
The photos show hedenbergite (Fe-rich calic clinopyroxene) with microcline. The clinopyroxene has higher relief than the microcline and a slight greenish color (the difference is visible in PP). It also has poorly developed cleavage. In the XP view, the clinopyroxene shows 2nd order interference colors -- mostly mottled yellow-green; the microcline only first order grays and white. The microcline also displays well developed crosshatched twins in some grains and one set of subparallel twins in others.
The field of view is about 2.5 mm.
Sanidine and Plagioclase in a Rhyolite Porphyry
The photo above shows two large sanidine (K-feldspar) grains, each having a simple twin down the center. Below the right hand sanidine there is a grain of plagioclase with faint twins visible (XP). These phenocrysts are surrounded by a fine grained groundmass of K-feldspar and quartz.
This specimen comes from Chaffee County,
Colorado. The field of view is about 2 mm.
Myrmekite and Perthite in a Granite from Colorado
The photos above show myrmekite (wormy
texture in XP view) and microcline (wavy black-gray-white twinning
in XP view). Both are characteristic of K-feldspar and are indistinguishable
in PP light. Myrmekite is an intergrowth of quartz
and feldspar. In this sample, the microcline twinning
does not produce a cross-hatched appearance, instead haviing
a wavey/spindly appearance. Minor biotite
(dark brown to tan) is present along the edges of the photo.
Micorcline in a Granite from Near Quincy, Massachusetts
Because it commonly exhibits both twinning
and exsolution, K-feldspar may show
complicated patterns when viewed under XP light. These photos
show mostly microcline. The large grain in the center displays
microcline (tartan plaid) twinning
but the twinning is not as well developed
as in some microclines. The K-feldspar grain above it, on the
edge of the photo, contains fine exsolved
lamellae of albite (visible as
stripes in XP). Minor plagioclase
is also present, and also a few small patches of green biotite.
K-feldspar, Quartz and Biotite in a Granite
This view shows two large flakes of brownish green biotite and minor hornblende (darker smaller grains) near the left edge; the hornblende does not have a distinctive appearance. The biotite has a pebbly texture and micaceous cleavage. The right half of the view contains a large grain of clear to cloudy K-feldspar. The clear grains with low birefringence left of the K-feldspar are quartz, and minor plagioclase (feldspar) is in the extreme lower left. The feldspars and quartz are difficult to distinguish in the PP view, although the feldspars are a bit cloudier due to incipient alteration. In the XP view, poorly developed microcline twinning gives the K-feldspar an irregular mottled appearance, clearly distinguishing it from the quartz.
This sample is a granite from the Minnesota
River Valley, Minnesota. The field of view is about 2 mm across.