Feldspar Gems and Minerals
Most of us have heard of moonstone and labradorite, but did you know these are part of a larger, very common group of minerals that represent the most common minerals on the earth’s surface? Yes, these are both feldspars and part of the feldspar mineral group that includes several semi-precious gems. When considered all-together, they represent the most common mineral group on the earth’s surface. Even though this group of minerals is common, several not so common gemstones also occur as a part of this group. Although quartz is more common in the earth's crust than any individual feldspar, the feldspar group combined exceeds quartz in abundance. With the exception of the ultramafic and ultrabasic rocks, such as peridotite and kimberlite, nearly all crystalline rocks have feldspar.
In addition to the pure end members, feldspar also includes solid-solution mixtures that are known as (1) alkali feldspars and (2) plagioclase feldspars. The alkali feldspars have chemical compositions that vary from potassium (K)-rich to sodium (Na)-rich and include some not so common compositions that are barium (Ba)-rich feldspars known as celsian and hyalophane. The pure end members of the alkali feldspars include orthoclase, microcline and albite. Microcline has the same chemistry (KAlSi3O8) as orthoclase, but is the lower temperature triclinic polymorph, whereas orthoclase is monoclinic. These minerals are typically found in potassium-rich igneous rocks; microcline is often found in granite and orthoclase in rhyolite. The solid-solution alkali feldspars include:
The plagioclase feldspars also form solid-solution compositions, or mixtures, that range from calcium- to sodium-rich. These solid crystals were once liquid chemicals in a magma that mixed and cooled until they crystallized to form a group of triclinic plagioclase feldspar crystals with varying amounts of calcium and sodium. These include:
As rock hounds and prospectors, we generally are not concerned with chemistry of all of these feldspars because most of us do not have a way to test their compositions. But geologists and mineralogists can use electron microprobes as well as petrographic microscopes to determine chemistry. Just remember orthoclase is typically found in granite, sanidine (the high temperature variety) is usually found in rhyolite, albite is often found in granites, quartz monzonites and latites and anorthite and bytownite are often found in basalts and gabbros. Labradorite, which often produces a beautiful gem, is typically found in an uncommon feldspar-rich rock known as anorthosite.
Two feldspars are popular with gemologists and rock hounds because they sometimes yield attractive, semi-precious gemstones: (1) albite, which can occur as MOONSTONE or as SUNSTONE; and (2) LABRADORITE, which often produces a beautiful color or fire.
One of the most attractive of the feldspar gems is moonstone. Most moonstone gems are colorless to white, semi-transparent to translucent, and characterized by undulating bluish color confined to a restricted angle of view. Essentially all moonstones are a solid solution mixtures of orthoclase and albite with the moonstone effect (termed adularescene) due to albite occurring in favorable oriented positions within orthoclase. Most moonstones are light blue, but some other varieties include white, green, brown to almost black. Although rare, some exhibit chatoyancy and produce cat’s eye moonstones and others may produce star moonstones. Some moonstones represent the more valuable feldspar gems and these are thought by some to bring good fortune.
Another gemstone found in the feldspar group is known as sunstone. Sunstone a variety of the usually white albite feldspar; however, the sunstone variety will be reddish to a shiny, golden feldspar with adularescence. When cut and polished to produce a cabochon, a billowy sheen will appear to move over the crest of the stone as it is rotated.