Originally Presented to the Georgia Mineral Society's Mineral Section Meeting
|Orthoclase||Amazonite (blueish crystals)|
The word feldspar literally translates to: feld, meaning field, and spar, meaning "easily cleaved material". It refers to any of several crystalline aluminosilicate minerals found in abundance in the earth's crust.
Minerals in the feldspar group are found in many places throughout the world, in pegmatites, even meteorites, and pass through all rock and ore-forming processes: igneous, metamorphic, hydrothermal and sedimentary, yet only rarely do they occur as gemstones. Feldspars are the most diversified and the most extensively investigated minerals on earth, though they only recently commanded the attention they deserve. It wasn't until the middle of the eighteenth century that minerals in this group were even vaguely mentioned in Wallerius's popular work "Mineralogy" written in 1753. They were briefly described as varieties of a "spar dense and lustrous" distinguished only by their colors of white, gray, and the occasional red. It wasn't until the end of the 18th century that serious crystallographic and chemical research began. Today, at least 40 varieties of feldspars have been identified, with a dazzling array of colors and distinctive features.
Feldspars have a specific gravity which varies between 2.5 to 2.7 depending on the chemical composition. The hardness ranges from 6 to 6.5. Its fracture, other than along the cleavage planes, is subconchoidal to uneven. Most feldspars crystallize from a melt in igneous rocks. Their crystals are tabular, flattened and most are complexly twinned.
All feldspars have certain physical characteristics in common: they have 2 good easily developable cleavages, one which is perfect, usually known as the basal plane. Here, the luster is pearly and the from the cleavage cracks parallel to this face, brilliant iridescent colors are sometimes reflected. The second cleavage, less perfect than the first, is parallel to the pair of faces which truncate the acute sides of the prism. The cleavages of monoclinic and triclinic feldspars are essentially the same with the following exception: the monoclinic crystals have cleavage planes that are exactly perpendicular to each other, giving them the name "orthoclase" which means "cleaving at right angles." In triclinic feldspar, the two cleavage directions are not quite at right angles. These are referred to as "plagioclase", meaning obliquely cleaving.
The presence of lamellae, which are thin, platelike layers within the crystalline structure, gives rise to the "Schiller Effect". This is an iridescence caused by the scattering of light between the layers. In Labradorites, the Schiller Effect is best developed, creating a lovely color play in shades of green, blue, gold and yellow. The color may be homogeneous or vary within a single feldspar crystal. Research is currently being conducted on lower quality plagioclase feldspar mined in Arizona to see if gamma radiation will produce a color shift or enhancement. Initial tests suggest this possibility exists.
POPULAR STONES IN THE FELDSPAR GROUPAMAZONITE
CRYSTAL SYSTEM: Triclinic
COLOR: Light green, blue-green or bluish with a mottled appearance and sometimes a fine crisscross network of light striations (which help to distinguish it from certain jades and beryls.)
Originally called "Amazonstone" because it was found near the Amazon River, this term was also used to describe Nephrite.
GENESIS: It is found in metamorphic rocks, intrusive magmatic rocks and pegmatites. The finest examples of Amazonite in the United States are found in Amelia Courthouse, Virginia. Pikes Peak, Colorado also boasts a variety of Amazonite found in cavities in a coarse pegmatite granite with smokey quartz crystals, often of huge size.
LORE: Worn by gamblers to attract money. It was also used by anyone taking a chance to ensure success.
CRYSTAL SYSTEM: Triclinic
COLOR: Blue, green, yellow are the most common colors, with copper-red being the most rare.
The most magnificent of all the feldspars, Labradorite occurs on the coast of Labrador, Canada. Although it is often a dingy, dark gray, the brilliant play of colors and unexpected flash combine to make this a most remarkable stone. It is a lime-soda-feldspar, comprised of approximately 55% silica, 25% alumina, 2% ferric oxide, 11% lime, 4% soda, with a touch of Potash. Feldspars with this composition are referred to as Labradorite, whether they come from Labrador, Madagascar, Scandinavia, or the United States.
The stone is cut, not with facets, which would destroy its reflected rays of color, but either perfectly flat or with a slight convex surface. It must be cut parallel to the reflecting surface, or no play of color will be seen. The sudden appearance and disappearance of brilliant colors is its most striking feature and gives us the word "Labradorescence".
An interesting specimen of Labradorite from Russia displays a perfectly recognizable image of Louis 16th. The head is of the finest azure-blue and stands out from a golden-green background. It is topped by a beautiful garnet-red crown with a border of rainbow colors and a small, silvery shining plume. In 1799 the owner received one quarter of a million francs for it.
It was customary to use Labradorite in the representation of objects with a metallic color such as the iridescent wings of butterflies. In the beginning of the 19th century, reliefs of Mandrill baboons were very much in vogue, and Labradorite was used to color the snouts of these most colorful animals.
GENESIS: Widely distributed throughout the United States, it is found in great abundance in Lewis and Essex Counties in New York in situ as boulders in glacial deposits. These boulders can be traced all the way down to Long Island and New Jersey, and were so numerous in one river in Lewis County, it was named "Opalescent River". Large quantities were quarried at Keesville, NY for monumental and building purposes. It is found in Pennsylvania, Arkansas and North Carolina, but the gem quality variety is only found in Labrador, Canada and Finland.
LORE: In ancient times, Labradorite was considered a general "good luck" stone. In recent times, it has become popular among New Age devotees as a spiritual, psychic-enhancing gemstone.
CRYSTAL SYSTEM: Monoclinic
COLOR: Almost colorless, tinged with yellow, whitish to silvery white with a blue shimmer.
Moonstone refers to the colorless, translucent, or almost perfectly transparent feldspar which in a certain direction reflects a bluish, milky light that has been compared to the light of the moon. It has also been called "girasol", "fish-eye", "wolf's-eye", "Ceylonese Opal" and "Water Opal". Very good glass imitations of moonstone are frequently used in expensive jewelry. These "fakes" are denser and less hard, and are only singly refracting, whereas real moonstones are distinctly doubly refracting.
GENESIS: The best moonstone is found in Sri Lanka, and is often referred to as the "National Stone of Sri Lanka". It is collected by hand by miners who dig deep, narrow holes in the earth. They lower themselves by rope to the bottom of these pits, fill their wicker baskets with loose dirt and gravel, and hoist the baskets up to the surface. It is then washed by hand and the gem quality moonstones are picked out of the gravel. The finest specimens in North America come from Allen's Mica Mine in Amelia Courthouse, Virginia.
LORE: It is said to counteract the negative influences of the number 13. Amulets of moonstone were hung in fruit trees to produce abundant crops. It was thought to protect against wandering of the mind, insanity and epilepsy. It was attributed to improving physical strength and reconciling lovers. If held in the mouth, a moonstone was supposed to help decide matters. It was even used to hypnotize people. In the Orient, moonstone was believed to be the solidified rays of the moon, and the glimmering light within was the light of the good spirit that lived in the stone. Occasionally, under magnification, a peculiar flaw appears: a long inclusion resembling a centipede.
CRYSTAL SYSTEM: Monoclinic
COLOR: Mid to golden yellow, it is perfectly transparent with a vitreous luster. Noble Orthoclase is most commonly faceted into a "Step Cut" and the gems are usually free from inclusions.
A somewhat rare variety of transparent orthoclase, Noble Orthoclase is nonetheless not considered valuable. It is sought by collectors and connoisseurs for their collections but is virtually unknown to the casual gem collector.
GENESIS: It is mainly found in pegmatites in Madagascar.
CRYSTAL SYSTEM: Triclinic
COLOR: Specimens are commonly colorless or straw-yellow, but some rare crystals have areas of red and/or green coloration. These vary from pale to intense and may contain zones of red, green, schiller or any combination of the three. The schiller consists of round, thin, extremely reflective platelets that are opaque to dark brown. Inside the crystal they appear pink but near the surface have a white, metallic reflection.
Known for their transparent, gemmy quality, sunstones at one time were believed to contain metallic copper. Very thin scales of hematite are arranged parallel to the direction of perfect cleavage. The glittering sheen of the sunstone is due to the reflection of this brilliant red metallic light from the surface of these scales. At the beginning of the 19th century, sunstone was considered a great rarity and was priced accordingly. Only a few small pieces were known, and they came from Sattel Island in the White Sea.
GENESIS: Sunstone is found in a basalt flow near the Rabbit Hills in Lake County, Oregon, as well as Siberia, Norway, and Statesville, North Carolina. A rare variety of green sunstone is found in Media, Pennsylvania.
LORE: No myths or folklore can be found in past history regarding the sunstone, but current New Age thought links it to protective energy. It is said to lend extra physical energy in times of stress or ill health.
CONCLUSIONFeldspar, in all its chemical compositions, habits and colors, is a fascinating mineral group to study and collect. In its most mundane usage, it is ground up for a polishing agent in toothpaste. In its highest and most noble form, it is faceted as a rare and beautiful gem. It is at once simple and sublime. Ralph Waldo Emerson once wrote: "A man is like a bit of Labrador Spar, which has no luster as you turn it in your hand until you come to a particular angle, then it shows deep and beautiful colors."
Lapidary Journal, December 1994: "SRI LANKA'S GLITTERING GRAVEL" by Stork Halstensen
Rock & Gem, August 1994: "PIKES PEAK PEGMATITES" by Steve Voynick
SIMON & SCHUSTER'S GUIDE TO GEMS AND PRECIOUS STONES, Simon & Schuster: NY, 1986
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