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(Ps-s-s-st...... They're Everywhere)
Louis Eldridge

Originally published in Cobb-L-Stones, Vol. XXIII, No. 4, April 1993<

The feldspar family of minerals presents somc of the most diverse, desirable and strikingly beautiful gemstones on earth. They range from colorless moonstone, through blue-green amazonitite (with rare reds), champagne orthoclase up to parrot-like colors of spectrolite and the dark gray blue-blacks of labradorite.

When we consider that feldspar comprises some 3/5 of the mineral content in the crust of the earth, it is easy to accept the diversification of its family characteristics.

Among other factors, the occurrence of feldvpar appears to center around the affinity of silicon to alter as a silicate with a wide variety of available elcments. It combines with 0, AL, K, NA, CA, Mg, Fe, SR, Ti, BA, Pb, and Ce, to form in either the monoctinic or triclinic system. The most important of these combinations are with 0, AL, K, NA, CA, and BA.

The common feldspars arc solid solutions of their components and are classified into three general groups:

  • (1) Orthoclase

  • (2) Microline

  • (3) Plagioclase

(1) Orthoclase (K, Al, Si3, O8,): This variety has short prismatic crystals, elongated on the "A" axis, or elongated parallel to the "C" axis. Crystals are frequently twinned at times with a penetration twin. They are usually found in coarse two granular masses that are cleavable. They are less frequently found in fine-grain, massive or cryptocrystalline rock formations. The crystals are seldom multiple twinned and striations on these crystals do not generally occur. Orthoclase has perfect cleavage and has a vitreous luster. Its color ranges from colorless to white, gray or flesh-rcd, salmon, yellow or brown. It produces a white streak.

When the crystal forms as a prism, it may display an opalescent play of colors, and if so, will be called moonstone. This feature is produced by microscopic to smaller size inclusions of "soda-rich feldspars," and is referred to as adulrescence. The stone is frequently light to transparent and may show itself in a variety of pastel colors. Most Orthoclase results from mid-tempcrature range occurrence.

If the orthoclase results from a high temperature action, it may result in the variety known as sanidine. This is not uncommon in volcanic activity. Possibly due to the intensity of the heat, sodium may have replaced as much as half of the potassium content. It results in glassy clear to light gray to brown transparent stone. The crystals may be tabular and may produce an exquisite stone if faceted. Sanidine will glow blue-white when subjected to gas flame.

The orthoclase variety named hyalophane, (K, BA)(Al, Si)2 Si2 08, develops if barium replaces part of the potassium. Another variety named celsian BA (Al2 Si2 08) may also develop if barium replaces more potassium in the stone.

Orthoclase is characterized by its cleavage, hardness, color, and is monoclinic. It has a basic 90° (+ or -) cleavage and generally has no twin striations on its best cleavage plane. It has a fusible difficulty of five and is insoluble in acids. It is not fluorescent after being subject to the blowpipe.

Orthoclase is a mineral that causes rocks to form in igneous and plutonic activity. Its mineral specimens and individual crystals are usually found in veins and porphyritic rocks. It is also found in granites, sycnites, nephelinc syenitcs, and may be found in arkos and conglomerates as well as in metamorphic rocks and in gneisses. It is rarely, if ever, found in pegmatites.

The term Orthoclase refers to its right angle cleavage and feldspar uses the German word feld (field) in combination with the term spar to complete the name.

(2) Microcline K(ALSI308) - is the only feldspar family member of its type. It is usually green to blue-green, white, red-brown or flesh colored. In rare instances it has been found as red. It is usually found in pegmatitcs.

This feldspar is triclinic and has some basic features identical to Orthoclase. It may be twinned on the "A" and "B" axis, so that the lamelle are at a basic 90° angle. This can create a grating characteristic on the stone.

Microcline has a perfect cleavage, ranges from six to 6 1/2 on the MOHS SCALE, has uneven fractures, is opaque and has a weak olive green florescence. It is usually cut "en cabochon" and is used primarily in necklace making and for ornaments. When cut as a cabochon, it may display a silver, gold or blue sheen if it contains a foliated structure and is cut in accord with the proper plane.

Microcline crystals may grow to unusually large dimensions. A mass with over 2000 tons weight, showing the features of an individual crystal, was found in a pegmatite in Russia at Karelia.

If microcline is found in close growth association with quartz, it forms graphic granite. During the formation of microcline, if potassium is forced out and is exceeded in the stone by a higher sodium content, the resulting stone is called anorthoclase.

Microcline differs from the triclinic plagioclases in that it has no twin striations on the principal face. Only deep green microcline is referred to as Amazon Stone (or amazonite). The name refers to the fact that this stone was first found adjacent to the Amazon River.

Excellent near faceting gcm quality microcline is found in Amelia County, VA. References related that the best green crystals are found in pockcts in schist near Crystal Peak and Pike's Peak in Colorado.

(3) Plagioclase feldspars are referred to as "soda like feldspars." They form from near 100% sodium pure albite to near 100% calcium pure anorthite. The theoretical divisions are:

NAME %Albite %Anorthite
Albite NA(AL SI3 ,O3)100 to 900 to 10
Oligoclase90 to 7020 to 30
Andesine70 to 50 30 to 50
Labradorite 50 to 3050 to 70
Bytowinite30 to 1070 to 90
Anorhite CA(AL2 Si2, 08)10 to 090 to 100

Albite through Anonhile, in the Plagioclase feldspar series have similar habits to their Othoclase cousins. They have stones that display adularesence and are called moonstone and sunstone. Their colors range from gray, greenish, bluish, reddish, yellowish, to colorless. Twinning is frequent in crystals and at times striations are so diminished that they are barely discernible, while on others, they arc quite pronounced. The direction of the striations varies contingent on composition.

Cleavage is perfect and the luster is vitreous to pearly. The stones are translucent to transparent.

Exceptional play of color is demonstrated by andesine and labradorite. Unique combinations of parrot-like tropical colors arc found in spectrolite, the newest of the family members.

Some of these stones are so similar that identification must be made by the polariscopc.

Bytowinite & anorhite are credited as being the most rare of the feldspars. They resemble a transparent labl-adoiile and seemingly offer few desirable traits for the time, effort, or cost to make them much in demand.

Feldspar offers a relatively economical ingredient for use in producing porcelain articles. It is pulverized and combined with quartz, clay and/or kaolin for that purpose. The glaze on porcelain products contains feldspar, as does the finish on many glass items. Feldspar is also used as a source for its constituent elements.

If you will take a moment to consider the everyday items we use in our daily lives that is produced with the elements that are found in the feldspars, you will recognize its importance and value.

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Dana, Minerals & How to Study Them, Dana (Rev: Hurlburt), J. Wiley & Sons, 1962;
Lapidary Journal, The Feldpars,, Charles J. Parsons, July 1967;
Matter, Ralph E. Lapp, Life Science Library, 1969; Text Book on Lithology, Jackson, McGraw-Hill, 1970;
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