PLAGIOCLASE

Plagioclase – NaAlSi3O8 – CaAl2Si2O8 – is a very common rock-forming tectosilicate, the second most abundant mineral in the Earth’s crust, just behind quartz. It is an industrial mineral with many uses.

“Plagioclase” is not actually a mineral, but a term used for the members of the solid solution between albite (Ab) (NaAlSi3O8) and anorthite (An) (CaAl2Si2O8), an isomorphic series. Its intermediate terms are oligoclase (90-70% Ab + 10-30% An), andesine (70-50% Ab + 30-50% An), labradorite (50-30% Ab + 50-70% An) and bitownite (30-10% Ab + 70-90% An).

Plagioclases are almost always twinned by [010] or perpendicular to {010}, resulting in polysynthetic twins that resemble a bar code. There are several other twinning laws, may be contact, single or multiple. May contain K and Mg. Epitaxies with microcline and orthoclase are possible. There are many varieties: albite (12), anorthite (7), oligoclase (2) and labradorite (3).

1. Characteristics

Crystal system: Triclinic pinacoidal.          

Color: White to gray, may be bluish, greenish or reddish.     

Habit: Tabular paralell to {010}. Granular, cleavable masses.       

Cleavage: {001} perfect, {010} good, {110} fair.       

Tenacity: Brittle.        

Twinning:  See above.      

Fracture: Irregular, conchoidal.       

Mohs Hardness: 6 – 6.5

Parting: No.         

Streak: White.         

Lustre: Vitreous, pearly.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.6 – 2.65 (albite)

 

2. Geology and Deposits

Except in some ultramafic, alkaline and very acidic rocks, plagioclases are essential constituents in igneous rocks, in many metamorphic rocks and also in immature clastic sediments.

Albite is typical of granites and granitic pegmatites, granodiorites, rhyolites, alkaline rocks (syenites, diorites and basalts), low-grade pelitic metamorphic rocks and hydrothermal veins. It can be autogenic.

Oligoclase is common in granites, granodiorites, rhyolites, andesite, dacites, syenites (nephelines), medium to high-grade metamorphic rocks (serpentinites, gneisses, etc.). Also detrital in sedimentary rocks.

Andesine is typical of intermediate igneous rocks such as andesites and diorites, but it also occurs in some alkaline rocks. Occur in high-grade metamorphic rocks of amphibolite facies. May be detrital.

Labradorite is the most common plagioclase of basic igneous rocks like gabbros, basalts and anorthosites, but also occurs in alkaline rocks like syenites. It is relatively rare in metamorphic rocks, but can occur in high-grade rocks such as amphibolites. Also detrital in sedimentary rocks.

Bitownite is typical of intrusive mafic igneous rocks (gabbros, anorthosites). In basalts it occurs as phenocrysts. In metamorphic rocks it is rare.

Anorthite is relatively rare; it occurs in basic and ultrabasic, intrusive and extrusive rocks. In high-grade metamorphic rocks (granite facies). In marbles and some meteorites.

 

3. Mineral Associations

Plagioclase occur with quartz, potassic feldspar (orthoclase, microcline, sanidine), micas (biotite, muscovite, chlorite), pyroxenes (augite, pigeonite, diopside), amphiboles (hornblendes, etc.), garnets, tourmaline, fluorapatite, zircon , rutile, corundum and many other minerals.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.528 – 1.533    nβ: 1.532 – 1.537     nγ: 1.538 – 1.542  (albite)

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. If altered to clay minerals, which is very common, it can be gray or brown.    

Relief: Low to very low.           

Cleavage:  {001} perfect, {010} good, {110} imperfect, {1-10} imperfect. The two main cleavages intersect at 86° angles, but are generally not visible in thin section.          

Habits: Typically long tabular (lamellar) crystals, sometimes very fine, may be skeletal (“swallow-tail”).            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Birefringence from 0.003 to 0.013, resulting in 1st order medium interference colors: dark gray to light gray, white.          

Extinction: Oblique, from 3o to 39o, depends on the chemical composition, the section, whether it is by cleavage or by twin. 

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

Twins: Common: Albite twin (polysynthetic), also Carlsbad and Pericline twin. Albite and Pericline together form a texture that resembles microcline, but the mesh is coarser and the lamellae better defined. Untwinned granoblastic albite is common in shales. Albite and anorthite tend to have single twins. Oligoclase has Albite twins with very thin lamellae; with increasing Ca content the twins become thicker.         

Zoning: Often shows zonation. It can be normal (Anorthite content decreases from the center to the edges), inverse (the opposite) or oscillatory (tending to normal, but with variations). The zonation crosses the polysynthetic twins.             

CONVERGENT LIGHT

Character: Albite, oligoclase, andesine and bitownite: B(+) or B(-).

Labradorite B(+), anortite B(-).          

2V angle: it is not diagnostic. Exsolution boards and strips can make the determination difficult.

albite: 76 – 82°, oligoclase: 82 – 90°, andesine: 76 – 83o , labradorite: 78 – 8 o, bitownite: 80 – 88o, anorthite: 78 – 83o         

Alterations: depends on the composition. Alters to sericite (mica), epidote, calcite, clay minerals (montmorillonite), scapolite, prehnite, zeolites and others. The most common processes, therefore, are sericitization, saussuritization, spilitization, calcitization and propylitization.          

May be confused with: the polysynthetic twins are extremely characteristic. The members of the isomorphic series of plagioclases are differentiated by the Methods of Michel-Levy (Rittmann) and Carlsbad-Albite. When plagioclase is very fine, granular, unaltered or without twins, it can be confused with other minerals.

Quartz does not alters and is U(+).

Nepheline alters, has no polysynthetic twins and is U(-).

Leucite and microcline have different twins, in which the dark bars taper towards both ends (“oleander-leaf”), which does not occur with the polysynthetic twins of plagioclases.

Cordierite is very similar but alters to pinnite and shows dark halos around radioactive inclusions (zircon), which do not occur in plagioclase.         

 

Plagioclase 01: In PPL, plagioclase in an anorthosite, a plutonic rock. Plagioclase crystals are colorless and with low relief, show no cleavage and cannot be identified. If the neighboring grains are other colorless minerals with low relief, like potassic feldspar and quartz, it is even difficult to see the grain's contour.
Plagioclase 01: The same section as in the previous image, now in XPL. Polysynthetic twins (“barcode”) are very diagnostic.
In XPL, small ripform grains of plagioclase in volcanic rock matrix. The twinning is still visible, but not so well-defined as in big crystals.
In XPL, large euhedral phenocrystal in volcanic rock. Around it, in the matrix of the rock, small plagioclases (gray/white) and clinopyroxenes (colored).
Altered plagioclases in XPL. With the development of sericite (white dots) the twins become increasingly more difficult to visualize. Plagioclase alters very easily to a number of other minerals.
In XPL, skeletal plagioclase phenocryst in volcanic rock. The grain is not complete, as it contains areas with the rock matrix (dark areas).
In XPL, plagioclases with a "swallowtail" texture, characteristic of crystals that have undergone very rapid growth in acidic volcanic rocks.
In XPL, arcosian sandstone with quartz grains (gray without alteration), potassic feldspar grains (gray with alteration) and plagioclase grains (with polysynthetic twins).
In XPL, plagioclase phenocryst in volcanic rocks (ryolites), with pronounced zonation, various types of twins and varying degrees of alteration. Each grain has a different appearance, but it is always a combination of twins and zonation.
In XPL, plagioclase phenocryst in volcanic rocks (ryolites), with pronounced zonation, various types of twins and varying degrees of alteration. Each grain has a different appearance, but it is always a combination of twins and zonation.
Image in XPL using the lowest magnification objective (2.5x). More or less radiating aggregate of fine acicular crystals of plagioclases and pyroxenes in basaltic rock. A term for this structure is “variolite”. Be careful not to confuse this structure with a spherulite resulting from volcanic glass devitirification (see volcanic glass sheet).
In XPL, plagioclase with complex twinning in volcanic rocks.
In XPL, plagioclase with complex twinning in volcanic rocks.
In XPL, plagioclase with complex twinning in volcanic rocks.
In XPL, plagioclase with complex twinning in volcanic rocks.
In XPL, plagioclase with complex twinning in volcanic rocks.

5. Reflected Light Microscopy

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

Sample preparation: the polishing of plagioclases offers no difficulties; it is similar to the polishing acquired by potassic feldspar and quartz.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like quartz and potassic feldspars. It is a lighter color than the one of pyroxenes, amphiboles and micas. 

Pleochroism:  No.     

Reflectivity: Very low (4%?).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is not visible.

Internal reflections: Generalized in light colors, tending to white.

May be confused with: many other light colored transparent minerals. Lamellar shapes are quite characteristic in volcanic rocks.

General Characteristics: 

Polysynthetic twins are only rarely visible, only when the twin planes are positioned at an especially favorable angle to the plane of the polished section.

Plagioclase 01: In PPL, , it is impossible to identify the plagioclase.
Plagioclase 01: The same section as of the previous image, now in XPL. Here, exceptionally, the twins became visible amid the clear to milky internal reflections. Generally this does not occur.
In some cases, polysynthetic plagioclase twin become visible in XPL as paralell colored bands, resembling calcite twins in XPL in Transmitted Light.
Plagioclase 02: In PPL, undersaturated volcanic rock. Silicates (plagioclases, feldspatoids) in dark gray colors. In the center of the image, pyrite (yellowish). On the left, magnetite grain (very light gray).
Plagioclase 02: The same section as of the previous image, now in XPL. Plagioclases laths with white internal reflections, magnetite is isotropic and pyrite has bright spots due to poor polishing.
Plagioclase 02:In XPL, detail of the same section as of the two previous images. Tabular plagioclase crystals with white internal reflections. Clear diffuse internal reflections correspond to feldspatoids.
Plagioclase 03: In XPL, plagioclase crystals in acid volcanic rock. Here, diagonally in the center of the image, plagioclase with clear internal reflections.
Plagioclase 03: The same rock as of the previous image. here with two plagioclase crystals perpendicular to each other.
Plagioclase 03: From the same rock as of the two previous images, here a plagioclase with swallowtail texture. Milky internal reflections are devitrified volcanic glass. Green colors are clay minerals.
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