ORTHOCLASE

Orthoclase – KAlSi3O8 – is a very common tectosilicate, an important rock forming mineral in various types of igneous rocks, as well as an industrial mineral with a number of uses.

It is a polymorph of sanidine, microcline and kokchetavite. Forms a series with celsian, which is a barium containing feldspar. It has six varieties (with As, with Ba, with pearly shine, with blue reflections, etc.) and can contain Ba, Rb, Ca, Na and Fe.

“Adularia” designates a low-temperature, more ordered variety of orthoclase, but can also be partially disordered microcline. Adularia usually occurs in paragenesis from alpine traction fissures, associated with hematite, fluorite, calcite, byssolite, quartz (rock crystal and smoky quartz), anatase, titanite, epidote and minerals of the Chlorite Group.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Colorless to white. May have green, gray, yellow and pink tones.

Habit: Short tabular to prismatic crystals. Granular, massive, cleavable masses.

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

Tenacity: Brittle.

Twinning: Common: Carlsbad, Baveno and Manebach laws.

Fracture: Irregular to conchoidal.

Mohs Hardness: 6

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

Streak: White.

Lustre: Vitreous, greasy.

Diaphaneity: Transparent.

Density (g/cm³): 2.55 – 2.63

 

2. Geology and Deposits

Orthoclase is a very common, medium-temperature alkaline feldspar that predominates in igneous rocks. It is frequent in silicic igneous rocks such as granites in general, granodiorites, diorites and syenites, particularly from shallow intrusions. It is also found in greisen, lamprophyres and ultra-potassic rocks. The high-temperature alkali feldspar – sanidine – is most common in volcanic rocks. The lowest temperature alkali feldspar – microcline – is more abundant in deep plutonic rocks.

Orthoclase can also be found in sedimentary rocks such as immature sandstones (arkoses, graywackes), as detrital grains. In other sedimentary rocks it can occur as an autogenic mineral.

In metamorphic rocks orthoclase occurr in some rocks of high-grade regional metamorphism and in rocks of contact metamorphism.

Potassic hydrothermal alteration can generate orthoclase.

 

3. Mineral Associations

Orthoclase is associated with minerals that commonly constitute granitoid rocks, such as quartz, plagioclase, micas (biotite, muscovite, chlorite), amphiboles (hornblendes) and pyroxenes (clinopyroxenes and orthopyroxenes), as well as accessory minerals such as epidote, zircon, apatite and titanite.

In granitic pegmatites it is associated with the typical minerals of this paragenesis, such as fluorite, beryl (aquamarine), garnet (spessartite), tourmaline and many others. It also occurs with aegirine.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.518 – 1.520   nβ: 1.522 – 1.524   nγ: 1.522 – 1.525

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: If unaltered it is completely colorless.

Very commonly it is altered, appearing “misty”, grayish, pale brown or brown due to alteration to clay minerals (kaolinite) and/or sericite (very fine-grained alteration muscovite).

May contain very fine inclusions of Fe oxides (goethite, hematite) that make it reddish in color.

Relief: Low, as quartz and plagioclase.

Cleavage:

{001} perfect and {010} good that cross at 90°, who are generally not visible on a thin section due to the very low relief. In the most central portion of the grain, the cleavage may be somewhat more visible.

Problems with the glue (epoxi, UV-glue) used in making thin sections can turn the cleavages visible, especially at the edges of the thin sections.

Habits: Xenomorphic (anhedral) or, less frequent, idiomorphic (euhedral) tabular (rectangular) grains. Very often perthites (plagioclase exsolutions) are present. Inclusions also are very common. Therefore, a grain of orthoclase can present, at the same time, a Carlsbad twin, perthites, inclusions and alteration to clay minerals and sericite.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.005 to 0.007 – 1st order medium interference colors: gray in various shades and white.

Extinction: Parallel in (010) sections, oblique (5 – 19o)  in (001) sections.

Elongation sign: Does not apply as orthoclase may be elongated in several directions.

Twins: Very frequent. It is usually the Carlsbad twin: a line (the twin plane) divides the grain into two portions, each one with its own interference color. Baveno, Manebach and Albite-Pericline twins may occur.

It never shows polysynthetic twins like plagioclase!

Zoning: Sometimes zoned, specially in volcanic rocks.

CONVERGENT LIGHT

Character: B(-)

2V angle: >80o

Alterations: very common, it is almost always altered. The fact that it is altered is important to easily differentiate it from quartz, which is never altered!

Argilization (or kaolinization): when it is changed to clay minerals (beige to brown colors to ND)

Sericitization: when it is altered to sericite (alteration muscovite mica = small luminous dots in CPL).

The alteration can be generalized or ocurr only in the center of the crystal or only along the edges.

May be confused with: other colorless minerals with low relief, without cleavage and low birrefringence.

Diagnostic are the facts of being colorless, the absence of cleavage, the low relief in PPL, the gray interference colors, some alteration, frequently perthites and Carlsbad twins.

Sanidine is very similar, but has a smaller 2V angle and usually occurs in volcanic rocks.

Quartz can be very similar, but it is never altered and is U(+).

Nepheline is similar, but occurs in other paragenesis and is U(-).

 

In the center of the image, in XPL, orthoclase grain with a white inclusion of quartz and exhibiting typical features: - anhedral - low relief - no visible cleavage - no twinning - somewhat altered - a little bit fractured
Orthoclase 01: In the center of the image, diagonally (NE-SW), approximately rectangular crystal of orthoclase in PPL. The more or less rectangular shape is very characteristic.
Orthoclase 01: The same section as in the previous image, now in XPL. The orthoclase is somewhat "cloudy", altered to clay minerals.
Several approximately rectangular crystals of orthoclase, deeply altered to clay minerals (brown colors), in PPL. This is kaolinization, very common for orthoclase.
In the center of the image, in XPL, almost vertical, tabular orthoclase crystal with Carlsbad twin and perthites (white almost horizontal lines). Around it, other similar grains.
In the center of the image, in XPL, orthoclase grain with pertites, which appear as lighter, somewhat irregular stripes, usually subparallel, arranged perpendicular to the grain elongation.
On the diagonal of the image, orthoclase in XPL with Carlsbad twin, which is the straight line separating the lighter portion from the darker portion. It is a very common feature.
In XPL, orthoclase crystals with and without Carlsbad twins, in syenite.
Orthoclase in XPL with Baveno twin (similar to an hourglass), a very rare feature.
Orthoclase in XPL with geometric quartz inclusions, forming the graphic texture. See literature to understand the feature.
Orthoclase in XPL with abundant approximately geometric shaped inclusions of quartz, characterizing the graphic texture. The quartz inclusions have the same optical orientation despite being in two different orthoclase grains (one is gray and the other is in extinction position, black). See specific literature to understand “graphic texture”.
Orthoclase 02: In PPL, in an acid lava flow, within a vein with idiomorphic quartz crystals, adularia crystals (altered - brownish) in a radial pattern.
Orthoclase 02: the same section as in the previous figure, now in XPL. Adularia is cloudy among quartz crystals.
Adularia crystals in PPL among idiomorphic quartz crystals in a vein in acid lava flow.

5. Reflected Light Microscopy

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

Sample preparation: polishing the orthoclase is not difficult and a good polish is usually achieved simultaneously with a good polishing of the associated quartz and plagioclase crystals.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, the same hue as quartz and plagioclase, but much lighter than amphibole and pyroxene.

Pleochroism: No.

Reflectivity: Low (<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  It does not show anisotropy.

Internal reflections: Generalized in light tones, which can be from whitish to yellowish, pink or very red.

May be confused with: other transparent minerals of light or whitish colors, when showing reflections in these colors. When with pink/red reflections, it is easy to recognize.

General Characteristics: 

Carlsbad twins sometimes is possible to recognize.

Multicolored internal reflections may appear depending on the angle of the cleavage relative to the plane of the polished section.

Orthoclase 01: in PPL, orthoclase crystals in a syenite. It is not possible to identify orthoclase in PPL.
Orthoclase 01: the same section as in the previous image, now in XPL. Now it is easy to recognize de tabular crystals, the Carlsbad twins and the internal reflections in pink-orange.
Orthoclase 02: in PPL, biotite (lamelar form in lighter gray at the bottom left), quartz and orthoclase (indistinguishable).
Orthoclase 02: the same section as in the previous image, now in XPL. Biotite is dark, quartz milky and orthoclase in shades of pink, can be orange or red.
In XPL, fluorite (top) with deep blue, almost black internal reflections, quartz with clear to white reflections, and orthoclase with orange internal reflections.
In XPL, diagonal in the image, tabular orthoclase crystal with Carlsbad twin and multicolored reflections in parallel alignments, thanks to an especially favorable angle of cleavage in relation to the plane of the polished section.
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