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
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.
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.