K-feldspar (orthoclase, microcline, sanidine and related varieties)



Occurrence
K-feldspar is widespread and common in a wide variety of igneous and metamorphic rocks and in some sedimentary rocks. Its chemistry and atomic arrangement are variable, so optical properties vary as well. The three principle varieties of K-feldspar are microcline (low temperature), orthoclase (medium temperature), sanidine (high temperature). Other, rare, related feldspar varieties include adularia and anorthoclase. Microcline is found in silicic to intermediate plutonic rocks (not volcanics) and some metamorphic rocks. Orthoclase is common in many silicic plutonic rocks, less commonly in volcanic rocks. It is also found in metamorphic and some sedimentary rocks. Sanidine is the common K-feldspar in silicic volcanic rocks.

Composition
The most significant end members are:
     ·orthoclase, KAlSi3O8
     ·albite, NaAlSi3O8

Identification
In general, keys to identifying K-feldspar are its (lack of) color, its low birefringence, and its twinning. In thin section, microcline, orthoclase and sanidine are distinguished by their twinning, optical sign, and 2Vo.

Important properties
 ·Appearance and habit - Microcline and orthoclase are generally anhedral; sanidine is more likely to form euhedral phenocrysts. K-feldspar crystals are typically tabular and often elongate. Exsolved Na-plagioclase may give them a wormy or ribbony appearance (perthite). Although there are two good cleavages, they may not be well displayed.
 ·Color - colorless, but may be altered giving it a grainy or grayish appearance. Orthoclase is frequently cloudy due to incipient alteration; sanidine is generally clear.
 ·Interference colors - birefringence is low, typically only white to gray interference colors.
 ·Twinning common - Microcline is often characterized by a distinctive plaid (cross-hatched) twinning with twin domains that pinch and swell. Orthoclase and sanidine may display simple twins that divide grains into two domains. When untwinned, distinguishing the different K-feldspar varieties may be difficult.
 ·Intergrowths - Perthite is created when an original high temperature feldspar unmixes (exsolves) into two compositions. Perthites grains consist of zones of Na-rich plagioclase alternating in complicated patterns with zones of K-feldspar.
 ·Interference figure - Obtaining an interference figure for microcline is difficult due to twinning. Orthoclase is biaxial (-) with a large 2V. Sanidine is biaxial (-) with a very small 2V (pseudouniaxial sometimes).

Similar minerals:
 ·K-feldspar is similar to quartz, but quartz lacks cleavage, lacks twinning, does not alter, exhibits undulatory extinction, and is uniaxial.
 ·K-feldspar can be difficult to distinguish from plagioclase. None of the K-feldspars develop parallel sided polysynthetic twins, but sanidine and orthoclase both may show simple twinning that appears similar to plagioclase. Microcline often shows distinctive "tartan plaid" twinning, but plagioclase may have two sets of twins yielding a similar appearance. In plagioclase, the contacts between twins tend to be sharper, and the twins do not pinch and swell.
 ·K-feldspar is similar to quartz, but quartz lacks cleavage, lacks twinning, does not alter, exhibits undulatory extinction, and is uniaxial.


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.