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Feldspars are a group of closely related minerals that together make up the majority of the Earth's crust. All of them have a hardness of 6 on the Mohs scale, so any glassy mineral that's softer than quartz and cannot be scratched with a knife is very likely to be a feldspar.
Feldspars lie along one of two solid-solution series, the plagioclase feldspars and the alkali or potassium feldspars. All of them are based on the silica group, consisting of silicon atoms surrounded by four oxygens. In the feldspars, the silica groups form rigid three-dimensional interlocking frameworks.01of 10
Plagioclase in Anorthosite
This gallery starts with plagioclase, then shows alkali feldspar. Plagioclase ranges in composition from NaAlSi3O8 to CaAl2Si2O8sodium to calcium aluminosilicates including every mixture in between. (more below)
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.
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.
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.
Plagioclase Feldspar in Anorthosite
Anorthosite is an uncommon rock consisting of plagioclase and little else. New York's Adirondack Mountains are famous for it. These are from near Bakers Mills.03of 10
The plagioclase variety called labradorite can display a dramatic blue internal reflection, called labradorescence.04of 10
Labradorite is used as a decorative building stone and has become a popular gemstone as well.
Potassium Feldspar (Microcline)
The polished "granite" (actually a quartz syenite) of a park bench displays large grains of the alkali feldspar mineral microcline. (more below)
Alkali feldspar has the general formula (K,Na)AlSi3O8, but varies in crystal structure depending on the temperature it crystallized at. Microcline is the stable form below about 400° C. Orthoclase and sanidine are stable above 500° C and 900° C, respectively. Being in a plutonic rock that cooled very slowly to yield these large mineral grains, it's safe to assume that this is microcline.
This mineral is often called potassium feldspar or K-feldspar, because by definition potassium always exceeds sodium in its formula. The formula is a blend ranging from all sodium (albite) to all potassium (microcline), but albite is also one endpoint in the plagioclase series so we classify albite as a plagioclase.
In the field, workers generally just write down "K-spar" and leave it at that until they can get to the laboratory. Alkali feldspar is generally white, buff or reddish and is not transparent, nor does it show the striations of plagioclase. A green feldspar is always microcline, the variety called amazonite.06of 10
Potassium Feldspar (Orthoclase)
Unlike the plagioclase group, which varies in composition, potassium feldspar has the same formula, KAlSi3O8. (more below)
Potassium feldspar or "K-feldspar" varies in crystal structure depending on its crystallization temperature. Microcline is the stable form of potassium feldspar below about 400° C.
Orthoclase and sanidine are stable above 500° C and 900° C, respectively, but they endure as long as they need to at the surface as metastable species. This specimen, a phenocryst from a Sierra Nevada granite, is probably orthoclase.
In the field, it usually isn't worth figuring out the exact feldspar you have in your hand. A true square cleavage is the mark of K-feldspar, along with a generally less translucent appearance and the absence of striations along cleavage faces. It also commonly takes pinkish colors. Green feldspar is always K-feldspar, a variety called amazonite. Field workers generally just write down "K-spar" and leave it at that until they can get to the laboratory.
Igneous rocks in which the feldspar is all or mostly alkali feldspar are called syenite (if quartz is rare or absent), quartz syenite or syenogranite (if quartz is abundant).07of 10
Alkali Feldspar in Granite Pegmatite
A pegmatite vein in a large commemorative boulder displays the excellent cleavage of alkali feldspar (most likely orthoclase), along with gray quartz and a little white plagioclase. Plagioclase, the least stable of these three minerals under surface conditions, is highly weathered in this exposure.08of 10
Potassium Feldspar (Sanidine)
A boulder of andesite from California's Sutter Buttes includes large grains (phenocrysts) of sanidine, the high-temperature form of alkali feldspar.09of 10
Alkali Feldspar of Pikes Peak
The pink granite of Pikes Peak consists predominantly of potassium feldspar.10of 10
Amazonite is a green variety of microcline (alkali feldspar) that owes its color to lead or divalent iron (Fe2+). It is used as a gemstone.