Plagioclase



Occurrence
Plagioclase (a feldspar) is widespread and common in a wide variety of igneous and metamorphic rocks and, to a much lesser extent, in some immature sedimentary rocks.

Composition
The most significant end members are:
     ·anorthite, CaAl2Si2O8
     ·albite, NaAlSi3O8

Identification
In general, keys to identifying plagioclase are its (lack of) color, its low birefreingence, and its twinning. When untwinned, plagioclase has bland features and may be difficult to identify unambiguously.

Important properties
 ·Color - colorless, but may be altered giving it a grainy or grayish appearance in PP light.
 ·Interference colors - birefringence is low, typically only white to gray interference colors, but occasionally first order yellow.
 ·Twinning common - may twin in a variety of ways. Twins are visible under XP light. Most diagnostic are polysynthetic twins giving a zebra stripe appearance. Less commonly, twins are "simple twins" resulting in only two twin domains in a grain, or are more complex, yielding stripes that go in different directions within a single grain.
 ·Intergrowths - Na-rich plagioclase may be intergrown with K-feldspar (microcline or orthoclase) creating a compound grain termed perthite.
 ·Appearance and habit - generally low to moderate relief; subhedral to anhedral crystals. May be euhedral in volcanics. Plates and lathes common in igneous rocks. Cross sections may appear rectangular and may show compositional zoning (XP). Two cleavages, but cleavage is often not well developed.
 ·Interference figure - biaxial (- or +), large 2V.

Similar minerals
 ·Plagioclase may be confused with quartz, but quartz lacks cleavage, lacks twinning, does not alter, exhibits undulatory extinction, and is uniaxial.
 ·K-feldspar (sanidine, orthoclase or microcline) can be difficult to distinguish from plagioclase. None of the K-feldspars develop polysynthetic twins, but sanidine and orthoclase both may show simple twinning similar to plagioclase. Microcline often shows distinctive "tartan plaid" twinning, but plagioclase may have two sets of twins yielding a similar appearance. In plagioclase, the contacts between twins tend to be sharp, while in microcline they are not.
 ·Cordierite may exhibit twinning similar to plagioclase but has slightly greater birefringence, often contains pleochroic halos around inclusions, and has poorer cleavage.


Plagioclase and Augite in a Basalt

The photos above show abundant plagioclase lathes (clear - PP; low order interference colors - XP) and the clinopyroxene augite (almost clear- PP; 2nd order yellow to blue to pink interference colors -XP) in a matrix that contains a lot of brown glass. Two vesicles are also present (clear, PP; isotropic, XP) -- one left of center on the bottom edge, and the other near the center of the right-hand side.

Some of the plagioclase exhibits twinning, giving a zebra striped appearance in XP light, although it is hard to see at this scale.

This sample is a basalt from the Cima Volcanic Field, southern California. The field of view is about 2.5 mm across.


Plagioclase and Augite in a Diabase

The photos above show mostly labradorite (a variety of plagioclase) with several grains of augite (a variety of clinopyroxene). The labradorite is clear and the augite has a typical dusty geen-gray color in PP light.

The plagioclase exhibits well developed albite twins, giving a zebra-striped appearance in XP light. Plagioclase, especially plagioclase in igneous rocks, has a tendency to alter to micas and clays over time. This sample is fairly fresh, but some plagioclase grains are slightly altered -- the alteration is visible as a sort of fine grained grunge on grains near the top center of the photo (in both PP and XP light). Note that the augite has significantly higher relief than the labradorite.

This sample is a diabase from the Palisades Sill, New Jersey. The field of view is about 2.5 mm across.



Plagioclase, Hornblende, Quartz and Biotite in a Gneiss from near Flin Flon, Manitoba

The clear minerals (PP) in this thin section are plagioclase (twinned) and quartz (not twinned). The green mineral (PP) is hornblende and the brown mineral is biotite (PP). The biotite is pleochroic, and some grains have a color similar to that of the hornblende. The two can be tough to distinguish, but biotite has a more flakey (micaceous) habit.

Note the complex twinning in the plagioclase; there are two sets of twins nearly perpendicular in the largest grain. In other grains the twinning is discontinuous.

The field of view is about 2.5 mm.


Plagioclase in Granodiorite from St. Cloud, Minnesota

The photos above show several large grains of plagioclase, a grain of faintly twinned K-feldspar, several grains of quartz, and several brown-green grains of biotite in a granodiorite from the Minnesota River Valley. In the PP view, the plagioclase, although clear, shows signs of incipient alteration (giving it a sort of rough scratchy appearance). In the XP view, the plagioclase shows zebra stripe twinning. The K-feldspar appears similar to the plagioclase but its twiinning is less well developed. The quartz, similar to both feldspars, shows undulatory extinction and no twinning.

The field of view is about 4.5 mm across.


Plagioclase in a Latite Porphyry from Wolf Creek, Montana

The photos above (PP on left, XP on right) show a few large plagioclase grains in a fine grained groundmass that is mostly plagioclase but also contains opaque magnetite (black in PP).

Besides being twinned, the larger plagioclase shows concentric compositional zonation (seen most easily in PP light).

The field of view is about 4.5 mm.


Zoned Plagioclase in a Dacite from near Helena, Montana

The large plagioclase grain shows concentric compositional zoning in XP light. It is surrounded by fine grained material that is mostly quartz and feldspar; a few grains of biotite can be seen in the upper right.

Field of view is about 4.5 mm.


Altered Plagioclase in an Augen Gneiss

Plagioclase often alters to a fine grained micaceous material. Sometimes the alteration is incipient, other times it may consume the entire mineral. Here it shows as dusting under PP light and as many highly birefringent specks under XP light.

The cluster of mafic minerals (brown and green) at the lower left is mostly hornblende with minor biotite (one obvious flake on the right of the mafic cluster). Some quartz is present just above the mafics; it is hard to see in the PP view, but can be distinguished by lack of twinning in the XP view.

This sample comes from the Adirondack Mountains, New York. The field of view is about 2 mm across.