Microcline

Microcline is polymorphous with Orthoclase and Sanidine. The only difference between them is their crystal structure. Microcline crystallizes in the triclinic system, and Orthoclase and Sanidine crystallize in the monoclinic system. Crystals of Microcline are generally much larger than those of Orthoclase, and a deep green color is tell-tale sign of Microcline, since Orthoclase does not exist in a deep green color. 

Microcline sometimes forms in association with Albite or other Plagioclase feldspar in alternating patterns, and forms a feldspar rock known as Perthite.

KAlSi₃O₈  Potassium aluminum silicate

 Microcline is a potassium-rich alkali feldspar (K.Al.Si3.O8), a tectosilicate mineral that is an important constituent of igneous rocks such as granites and pegmatites. It is triclinic and the crystal habitat is often characterized by crosshatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline.

Most often found in pegmatites, also in metamorphic rocks, hydrothermal veins, and sedimentary conglomerates.

Complex Tests

Soluble in hydrofluoric acid

Its fracture is conchoidal to uneven . Its cleavage is 2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º. 

It is vitreous , Specific gravity is 2.5 - 2.6

Crystal system is triclinic .

Microcline in a Granite

These photos show a large grain of microcline in a granite. 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 60^o and 120^o. A few grains of magnetite are in the lower right. The field of view is 2.5 mm.

Microcline and Hedenbergite in a Syenite

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.

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.

Myrmekite and Perthite in a Granite

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

As 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.

 Microcline with some albite

A thin section image showing microcline grain in a coarse-grained sandstone. It is characterized by cross-hatched (tartan) twinning in crossed polars. Width of view 3 mm.

Microcline with perthitic exsolution. White veins are composed of albite. Width of sample 43 mm.

Blocky white Microcline crystals to 25 mm long twinned following the Manebach law of twinning and easily visible to even a beginning collector. A 22 mm Smoky Quartz crystal is growing out to one side .

Blocky, tan-colored Microcline crystals with one small Smoky Quartz crystal.

Giant Microcline crystals associated with Schorl Tourmaline. These crystals are incredibly sharp and well-formed, which is especially remarkable considering their size. The crystals are associated with black contrasting elongated Schorl crystals.

Microcline var. Sunstone

Translucent cleavage of pink sunstone variety of microcline caused by microscopic parallel inclusions of hematite that reflect simultaneously (Schiller-effect). The sunstone effect is visible in the dark zones when illuminated from one direction.

Photograph of thin section microcline showing the grid structure of twining under cross-polarized light.

Microcline and minor franklinite

Very slight post-metamorphic K-metasomatism of metatonalite in the Mokgware Granite. Note relict plagioclase within the largest microcline grain.

Microcline

Natural specimen of microcline feldspar split along mineral cleavage planes, an important igneous rock - potassium-rich alkali feldspar.

Different types of K-feldspar-bearing (microcline) rocks: (A) a polished hand specimen (~20 cm in long direction) of granular microcline (K-Fsp) in intergrowth with charoite (Ch), (B) a large xenolith of a massive monomineralic microcline rock (K-Fsp) within charoitite (Cht) cut-block, (C) interlayering of white and dark green microcline veins (K-Fsp) with charoitite (Cht) in a natural outcrop. Reactional snow-white microcline is seen between massive microcline and charoitite in figure B. A hammer with 50-cm long handle is shown in figures B and C for the scale.

Sharp, off-white microcline crystals to 2.0 cm are scattered on the mounded matrix. The largest crystal is at the top. Pastel-blue albite crystals are richly sprinkled and epitactically grown on the microcline crystals. There is tiny, octahedral pyrochlore crystals that is set on the lower left of the specimen.

Amazonstone, a variety of green microcline.