Plagioclase tends to be more transparent than alkali feldspar; it also very commonly shows striations on its cleavage faces that are caused by multiple crystal twinning within grains. These appear as the lines in this polished specimen.
Pure Na[AlSi3O8] is called albite, and pure Ca[Al2Si2O8] is called anorthite. The plagioclase feldspars are named arbitrarily according to this scheme, where the numbers are percentage of calcium expressed as anorthite (An):
- Albite (An 0–10)
Oligoclase (An 10–30)
Andesine (An 30–50)
Labradorite (An 50–70)
Bytownite (An 70–90)
Anorthite (An 90–100)
The geologist distinguishes among the plagioclase feldspars under the microscope in the laboratory. One way is to determine the mineral's density by putting crushed grains in immersion oils of different densities. (Albite's specific gravity is 2.62, anorthite's is 2.74, and the others fall in between.) The really precise way is to use thin sections under the petrographic microscope, determining the optical properties along the different crystallographic axes.
The amateur has a harder time. Large grains of plagioclase like this specimen display two good cleavages that are off square at 94° (plagioclase means "slanted breakage" in scientific Latin). The play of light in these large grains is also distinctive, resulting from optical interference inside the mineral. Both oligoclase and labradorite show it, and in the latter it often has a dazzling blue hue called labradorescence. If you see that it's a sure thing.
The igneous rocks basalt (extrusive) and gabbro (intrusive) contain feldspar that is almost exclusively plagioclase. True granite contains both alkali and plagioclase feldspars. A rock consisting of only plagioclase is called anorthosite. A noteworthy occurrence of this unusual rock type makes up the heart of New York's Adirondack Mountains (see the next page of this gallery); another one is the Moon. This specimen, a gravestone, is an example of anorthosite with less than 10 percent dark minerals.