Feldspar Gems and Minerals
Most have heard of moonstone and labradorite, but did you know these are part of a larger, common group of minerals represented by the most common minerals on the earth’s surface? Yes, these are feldspars and part of the feldspar mineral group that includes several semi-precious gems. Although quartz is more common than any individual feldspar, the feldspar group combined exceeds quartz in abundance. With the exception of 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 granite, quartz monzonite and latite; and anorthite and bytownite are often found in basalt and gabbro. Labradorite, which often produces a beautiful gem, is typically found in an uncommon feldspar-rich rock known as anorthosite.
For our Wyoming rockhounds, there is a very large outcrop of anorthosite in the Laramie Mountains north of Laramie. Highway 34 cuts through the Laramie Mountains at Sybille Canyon and as you drive through this canyon, you will likely see small reflections from the canyon walls of sunlight bouncing off billions of feldspar crystals - hey, you are now in the Laramie Anorthosite Complex: 350-square-miles of anorthosite batholith containing billions of carats of labradorite gems. In this area, old quarries cut into the anorthosite exposing labradorite and other feldspars. Even road bed material used for highway 34 and some country roads (Albany County 11 and 12), contain gem-quality labradorite. If you are searching for gemstones, keep in mind there may be a giant iolite deposit at Ragged-Top and Sherman Mountain to the south near the 9th street road that leads northeast from Laramie. And some kimberlites have been identified in the anorthosite batholith as well as adjacent (east) of the batholith at Iron Mountain. Kimberlites are best known for diamonds, chromian diopside and pyrope garnet, as well as rattlesnakes. Yep, I found dozens of rattlesnakes in the Iron Mountain kimberlite district when I mapped it some years ago. So, keep your eyes peeled on the ground and watch for the snakes (no, not the politicians - their cousins) and search for gemstones. Some copper, gold, tungsten, ruby, sapphire, kyanite and even nickel has been identified and detected in the region.
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.