MICROCLINE

Microcline –  KAlSi3O8 – is a very common tectosilicate, an important rock-forming mineral of granitoid rocks. It is an important industrial mineral for the manufacture of porcelain, also ceramics and glass.

Microcline almost always is twinned according to various laws, which greatly simplifies its recognition under the microscope. More common are the polysynthetic twins according to the Albite and Pericline laws, forming a checkered pattern. There are also Carlsbad, Baveno and Manebach twins. But microcline can also occur without any twins.

The largest known crystal on the planet is microcline: 50 m x 36 m x 14 m (Devils Hole Beryl Mine, Colorado, USA). The microcline is a polymorph of orthoclase, sanidine and kokchetavite. May contain Pb, Rb, H2O, Cs, Li, Fe, Ca and Na.

A green to bluish green variety of potassic feldspar is called “amazonite” and is usually microcline; can be orthoclase. “Chesterlite” is a white, discreetly lustrous variety. “Ferruginous microcline” is a Fe rich variety. “Hyalophano” is a variety with Ba.

1. Characteristics

Crystal system: Triclinic pinacoidal.

Color: Gray to white, grayish yellow, yellowish, bronze, salmon pink, bluish green to green.

Habit: Massive, granular or as prismatic crystals. Perthites are very common.

Cleavage: {001} perfect, {010} good.

Tenacity: Brittle.

Twinning: See above.

Fracture: Irregular.

Mohs Hardness: 6 – 6.5

Parting: On {100}, {110}, {-110} and {-201}.

Streak: White.

Lustre: Vitreous, pearly on cleavages.

Diaphaneity: Transparent.

Density (g/cm³): 2.54 – 2.57.

 

2. Geology and Deposits

Microcline is the lowest temperature potassic feldspar (sanidine = high temperature, only volcanic rocks; orthoclase = intermediate term, higher temperatures than microcline) and is abundant in acidic plutonic igneous rocks such as granites, granodiorites, diorites, syenites and pegmatites that are associated with these rocks. May occur in lamprophyres.

In metamorphic rocks it is restricted to high-grade metamorphic rocks such as gneisses and granulites. Can occur in greisen. Microcline is relatively resistant to weathering and is therefore a common detrital mineral, forming grains in sands and occuring in immature sandstones.

 

3. Mineral Associations

Being a common mineral, microcline is associated with many other minerals: quartz (including the smoky quartz variety), micas (biotite, muscovite, chlorite), plagioclases (albite, clevelandite), amphiboles (hornblende), titanite, zircon, beryl (aquamarine), fluorite, tourmalines (schorlite), garnets (spessartine) and aegirine, among many others.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.514 – 1.529     nβ: 1.518 – 1.533      nγ: 1.521 – 1.539

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless; it never has color much less pleochroism. Due to the alteration to clay minerals, it can be cloudy, with yellowish or brownish tones.

Relief: Low, like other feldspars and quartz.

Cleavage: {001} perfect and {010} good, with an angle of 90.6º between them.

Due to the low relief of the microcline, these cleavages are usually not visible under a microscope.

Habits: Massive, tabular or in short prismatic crystals. Anhedral to subhedral grains, may present perthites and/or alteration with “clouds” of clay minerals (kaolinite).

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.005 to 0.008: first order medium colors, with grays in various shades (dark to light) to white.

Extinction: Oblique with angles of 15-20o oblique in (001), but this is difficult to measure due to the twinning.

Elongation sign: ES(-) in relation to cleavage, generally impossible to determine.

Twins: Twins in two perpendicular directions, forming a twinning in a pattern of  “parquet” or “chess”, which are intersections of albite and pericline twins.

Typically they do not form bands with constant thickness, but bands that end in spikes; they individually narrow and widen rotating the stage and, in four positions during the complete turn of the stage, disappear completely.

There may be single twins: Carlsbad, Baveno and Manebach. It may also have perthites: dissolved albite (plagioclase) form lamellae, irregular bands and drops. Low-temperature autogenic microcline may be without twins and is easily confused with other minerals.

Zoning: Sometimes zonation occurs.

CONVERGENT LIGHT

Character: B(-)

2V angle: 66 – 103o

Alterations: microcline is more resistant to weathering than other feldspars such as orthoclase, sanidine and plagioclase. As a result, it is still unaltered while the other feldspars in the rock have already developed alteration minerals (clay minerals, sericite). Due to this greater resistance to weathering, microcline occurs as grains in sandstones. Alteration forms clay minerals (kaolinite) and sericite (alteration mica, usually muscovite).

May be confused with: other colorless twinned minerals with low-relief.

Anorthoclase may have similar twins, but it only occurs in volcanic rocks (microcline never occurs in volcanic rocks).

Plagioclases with an albite-pericline twin may be similar, but the twin planes are well defined, do not narrow and widen rotating the stage, and do not all disappear in certain positions rotating the stage.

Leucite may have a similar twin pattern, but its relief is lower, the crystal are generally rounded and the birefringence is lower (the interference colors in CPL are much darker).

 

Microcline 01: Microcline in plutonic rock. Here are quartz, orthoclase and microcline crystals. All are anhedral, colorless, with low relief and without cleavage: it is impossible to differentiate the minerals from each other.
Microcline 01: the same section of the previous figure, now in XPL. Microcline has its typical checkered twins, potassic feldspars have other twins and/or are somewhat cloudy, while the quartz is unaltered and is U(+).
Microcline 02: in XPL, microcline with the typical checkered twins.
Microcline 02: the same grain of the previous image, just rotated slightly, no longer show the twins, but only an uniform interference figure. This is one of the characteristics of the microcline: as the stage rotates, there are four positions where you can see the twins alternating with four positions where you cannot see the twins.
In XPL, an important diagnostic characteristic of the microcline twins: unlike the polysynthetic twins of plagioclases, for example, the microcline twins do not have a constant thickness, do not cross the entire crystal and end on both sides in fine points.
In XPL, detail of the appearance of the twins of microcline. Plagioclase twins have uniform thickness, much different than the twins of microcline.
Two microcline grains in intensely silicified sandstone. Around them, grains of quartz. In the quartz grains, the fine line that marks the original contour of the grains and the overgrowth of silica can be seen. XPL.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of microcline. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with microcline.

Sample preparation: the polishing of the microcline is simple and follows the polishing of other feldspars and quartz.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, as quartz, plagioclase and other feldspars.

Pleochroism: No.

Reflectivity: Very low (<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is not observed.

Internal reflections: Clear generalized, milky to white in crystals that are macroscopically white. One should always expect internal reflections that are similar to the color of the mineral in hand specimen.

May be confused with: many other clear to white colored transparent minerals such as quartz, other feldspars, etc.

General Characteristics: 

Twins are only observable in very rare cases. It is the same situation as for plagioclases: polysynthetic twins are generally unobservable, but they are visible when the angle between the twin planes and the plane of the polished section is especially favorable.

Microcline in XPL, showing typical twins. It is a rare situation, which only occurs when the angle between the twins and the plane of the polished section is especially favorable. Generally, only generalized clear to white reflections are observed, which do not allow the identification of the microcline.
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