SANIDINE

Sanidine – (K,Na)(Si,Al)4O8 – is a common tectosilicate, a high-temperature potassium feldspar typical of volcanic and hypabyssal rocks. Only the rare colored varieties have any economic importance.

High-temperature sanidine (“high sanidine”) is stable at temperatures above 800°C and low-temperature sanidine (“low sanidine”) is stable between 650 and 800°C. Below 650ºC, orthoclase and microcline are formed. High temperature sanidine forms a series with high temperature albite and generally has more impurities (contents of other elements) than low temperature sanidine. Over geological time, sanidine is demixed and transformed into orthoclase, so sanidines from older volcanic rocks usually are present as orthoclases.

Sanidine is a polymorph of orthoclase, microcline, and kokchetavite (metastable polymorph found as inclusions in clinopyroxene and garnet). May contain Fe, Ca, Na and H2O impurities. Carlsbad twins are very common; Manebach and Baveno twins are rare and complex twins are possible.

As varieties there is an iridescent blue sanidine (“azulicite”), another with up to 5% BaO (“Ba-sanidine”) and “citron feldspar”, a yellow variety, gem quality, from Madagascar.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Colorless, white, gray, yellowish-white or reddish-white.

Habit: Tabular, with square basal section. Massive, granular.

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

Tenacity: Brittle.

Twinning: See above.

Fracture: Irregular, conchoidal.

Mohs Hardness: 6

Parting: On {100}

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 2.56 – 2.62

 

2. Geology and Deposits

Sanidine is the highest temperature potassium feldspar, orthoclase is of medium temperature and microcline is of low temperature. Sanidine generally occurs as metastable crystals in rapidly cooling volcanic rocks (lava and hypabyssal rocks), in the form of idiomorphic phenocrysts.

It is the normal potassium feldspar of intermediate to acidic volcanic rocks such as rhyolites, rhyodacites, trachytes, andesites, phonolites and tephrites.

Sanidine occurs in some intrusive granites at low depths and syenites. Also in lamproites and naturally in sanidinites. It also occurs in high temperature contact metamorphism rocks. As detrital grains it can be found in sediments.

 

3. Mineral Associations

In acid vulcanites sanidine occur associated with quartz, plagioclase, biotite, hornblende, orthopyroxenes, clinopyroxenes and volcanic glass.

In trachytes it occurs with plagioclase, biotite and aegirine-augite.

In phonolites with nepheline, aegyrin-augite and minerals from the Sodalite Group (sodalite, hauyna, nosean).

In addition, it is associated with magnetite, hematite, ilmenite, garnet (melanite), zircon and apatite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.518 – 1.525   nβ: 1.523 – 1.530    nγ: 1.525 – 1.531

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, never shows color much less pleochroism. May be cloudy (PPL: brown, brownish) due to weathering products (clay minerals and sericite).

Relief: Very low to low.

Cleavage: {001} perfect, {010} good, they intersect at 90º. Cleavages are usually not visible under a microscope, only if are problems with the glue of the thin section.

Very young and/or poorly altered sanidines show no cleavage or have poorer quality cleavages than older sanidines.

Habits: Typically tabular parallel to {010}, prismatic parallel to x-axis, with rectangular sections. The basal sections are square, rectangular, or have six sides. In volcanic rocks it is usually idiomorphic.

Through geologic time, sanidines develop exsolution compounds and transition to orthoclase.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.005 to 0.008, 1st order colors: black, various shades of gray to white. It never goes from white to 1st order yellow, unless the thin section is thicker than 30 microns.

Extinction: Oblique, with an angle of 5 – 9º, can be parallel.

Elongation sign: Not applicable as sanidine may be elongated along the x, y or z axis.

Twins: Commonly simple Carlsbad twins according to the plane of composition {010}, dividing the crystal into two segments. Plans {021} and {001} can also occur. Multiple planes of twins can make the crystal look like a plagioclase, but sanidine never has polysynthetic twins.

Zoning: Often zoned, which is expressed through variable birefringences and different extinction angles.

CONVERGENT LIGHT

Character: B(-)

2V angle: 0 – 47o

Alterations: alters to clay minerals and sericite. It is a very common and to some extent diagnostic alteration, as it easily differentiates sanidine from quartz. These changes make the sanidine cloudy in PPL rather than colorless. Sanidines in volcanic rocks from past geological times usually show partial or full transformation to orthoclase, which is the stable phase of potassium feldspar.

May be confused with: several minerals.

Quartz has no cleavage, is U(+) and never is altered.

Nepheline in volcanic rocks can be very difficult or even impossible to distinguish from sanidine, if the sanidine show a very small 2V angle and displays no Carlsbad twin.

Other potassium feldspars have much larger 2V angles.

Typical for sanidine is:

(i) occurrence as idiomorphic phenocrysts in fine matrix of intermediate to acidic volcanic rocks,

(ii) Carlsbad twins only, never polysynthetic twins (beware of narrow crystals, which may be plagioclase phenocrysts) and

(iii) turbidity (“cloudyness”) from incipient alteration to clay minerals. Remember that sanidines from older vulcanites are usually already transformed into orthoclase.

 

Sanidine 01: in PPL, a sanidine crystal in a volcanic rock. Typical is the tabular habit, low relief and the absence of cleavage.
Sanidine 01: the same crystal of the previous image, now in XPL. Gray interference colors are and the absence of polyshynthetic twins help to identify sanidine.
Sanidine in XPL in phonolite with typical tabular habit. In XPL it shows an almost parallel extinction.
In XPL, sanidine crystal with Carlsbad twin.
In XPL, sanidine with incipient zonation. Zonation is relatively frequent in sanidine crystals, but their development can be quite irregular.
Sanidine 02: in PPL, sanidine crystal with cleavage. The image was obtained with the diaphragm almost completely closed.
Sanidine 02: the same crystal as of the previous image, now in XPL, showing its idiomorphic form.
In PPL, a basal section of sanidine, with six sides. Basal sections can be square, rectangular or six-sided. The sanidine here is significantly altered and show zonation (transformation into orthoclase).
Another basal section of sanidine, with six sides. Basal sections can be square, rectangular or six-sided. The sanidine here is significantly altered and show zonation (transformation into orthoclase). PPL.
Sanidine 03: In rhyolite. in PPL, idiomorphic phenocrysts of quartz (very clear) and sanidine (almost indistinguishable from matrix).
Sanidine 03: the same section as of the previous image, now in XPL. Sanidine crystals show alteration, quartz is clear.
In XPL, in trachyte, cross twins formed by two individuals, each one twinned according to Carlsbad twin. These complex twins the literature generally does not mention much less describe.
In XPL, in trachyte, cross twins formed by two individuals, each one twinned according to Carlsbad twin. These complex twins the literature generally does not mention much less describe.
In XPL, in trachyte, cross twins formed by two individuals, each one twinned according to Carlsbad twin. These complex twins the literature generally does not mention much less describe.
In XPL, in trachyte, cross twins formed by two individuals, each one twinned according to Carlsbad twin. These complex twins the literature generally does not mention much less describe.
Twinned sanidine phenocryst in a sanidine-nephelinite. Despite its typical characteristics of nepheline, including a very clear uniaxial negative interference figure, it may be sanidine. Twins are much more expected in sanidine than in nepheline and it may be impossible to distinguish between sanidine and nepheline phenocrysts.

5. Reflected Light Microscopy

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

Sample preparation: sanidine has the same polishing behavior as plagioclases and other potassium feldspars. It is not difficult to polish, but there are always some irregularities in the polish.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray like quartz, other potassium feldspars, plagioclases, nepheline and feldspathoids.

Pleochroism: No.

Reflectivity: Low (<6%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.

Internal reflections: Generalized clear, colorless to white, can be multicolored if the grain is not altered.

May be confused with: nepheline, which has the same habit, occurs in the same paragenesis and has the same characteristics in Reflected Light. It can also be confused with other light colored transparent minerals like orthoclase, plagioclase, etc. There is usually no way to identify sanidine by Reflected Light alone.

Sanidine 01: Diagonally in the image, in PPL, sanidine phenocryst. The crystal is full of inclusions and has a Carlsbad twin, which is difficult to see.
Sanidine 02: The same section as of the previous image, now in XPL. The sanidine shows typical clear, colorless to multicolored reflections.
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