Feldspar
Feldspars (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.
Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock.
Etymology
Feldspar is derived from the German Feld, "field", and Spath, "a rock that does not contain ore". "Feldspathic" refers to materials that contain feldspar. The alternate spelling, felspar, has now largely fallen out of use.
Compositions
This group of minerals consists of framework tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:
Albite endmember NaAlSi3O8
Anorthite endmember CaAl2Si2O8
Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, or more properly plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth. Albite is considered both a plagioclase and alkali feldspar. In addition to albite, barium feldspars are also considered both alkali and plagioclase feldspars. Barium feldspars form as the result of the replacement of potassium feldspar.
Alkali feldspars
The alkali feldspars are as follows:
- orthoclase (monoclinic), — KAlSi3O8
- sanidine (monoclinic) —(K,Na)AlSi3O8
- microcline (triclinic) — KAlSi3O8
- anorthoclase (triclinic) — (Na,K)AlSi3O8
Sanidine is stable at the highest temperatures, and microcline at the lowest. Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.
Plagioclase feldspars
The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):
- albite (0 to 10) — NaAlSi3O8
- oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8
- andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8
- labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8
- bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8
- anorthite (90 to 100) — CaAl2Si2O8
Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solution are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.
Barium feldspars
The barium feldspars are monoclinic and comprise the following:
- celsian — BaAl2Si2O8
- hyalophane — (K,Na,Ba)(Al,Si)4O8
Feldspars can form clay minerals through chemical weathering.
Uses
- Feldspar is a common raw material in the production of ceramics and geopolymers.
- Feldspars are used for thermoluminescence dating and optical dating in earth sciences and archaeology
- Feldspar is one of several abrasive ingredients in Bon Ami, a brand of household cleaner in the USA.
- Potassic feldspars are used frequently in K-Ar dating and argon-argon dating, with applications to both geochronology and thermochronology.
In 2005, Italy was the top producer of feldspar with almost one fifth of world share, followed by Turkey, China and Thailand—reports the International Monetary Fund.
Varieties
1.Sunstone
Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a brilliant spangled appearance; this has led to its use as a gemstone. It has been found in Southern Norway, and in some United States localities. It is the official gemstone of Oregon.
Properties
The optical effect appears to be due to reflections from enclosures of red haematite, in the form of minute scales, which are hexagonal, rhombic or irregular in shape, and are disposed parallel to the principal cleavage-plane. These enclosures give the stone an appearance something like that of aventurine, whence sunstone is known also as "aventurine-feldspar." The optical effect called shiller and the color in Oregon Sunstone is due to copper. In the middle part of this crystal, it sparks a lot, and usually has a dark color in the middle, and the color becomes lighter as it becomes the outer part.
The feldspar which usually displays the aventurine appearance is oligoclase, though the effect is sometimes seen in orthoclase: hence two kinds of sunstone are distinguished as "oligoclase sunstone" and "orthoclase sunstone."
Distribution
Sunstone was not common until recently. Previously the best-known locality being Tvedestrand, near Arendal, in south Norway, where masses of the sunstone occur embedded in a vein of quartz running through gneiss. Due to the discovery of large deposits in Oregon, Sunstone is now readily available.
Other locations include near Lake Baikal in Siberia, and several United States localities—notably at Middletown Township, Delaware County, Pennsylvania, Lakeview, Oregon, and Statesville, North Carolina.
The "orthoclase sunstone" variant has been found near Crown Point and at several other localities in New York, as also at Glen Riddle in Delaware County, Pennsylvania, and at Amelia Courthouse, Amelia County, Virginia.
Sunstone is also found in Pleistocene basalt flows at Sunstone Knoll in Millard County, Utah.
Oregon sunstone
A variety known as Oregon sunstone is found in Harney County, Oregon and in eastern Lake County north of Plush. Only Oregon sunstone contains inclusions of copper crystals. Oregon sunstone can be found as large as three inches across. The copper leads to varying color within some stones, where turning one stone will result in multiple colors. The more copper within the stone, the darker the complexion.
On August 4, 1987, Oregon State Legislature designated Oregon sunstone as its state gemstone by joint resolution.
One of the leading mining operations is the "Dust Devil Mine", founded by rockhounds and which started selling Oregon sunstone in 1996. Many award winning gem cutters and carvers dig their own sunstone at the mine before cutting and carving them. Stones of over 300 cts have come from the pit.The largest producing Oregon Sunstone mine is the Ponderosa Mine north of Burns, Oregon.
2.Orthoclase
Orthoclase (endmember formula KAlSi3O8) is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.
Formation and subtypes
Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite.
Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi3O8). While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite.
The higher-temperature polymorph of orthoclase is sanidine. Sanidine is common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks, and is notably found in trachytes of the Drachenfels, Germany. The lower-temperature polymorph of orthoclase is microcline. Adularia (from Adular) is found in low temperature hydrothermal deposits. The largest documented single crystal of othoclase was found in Ural mountains, Russia. It measured ~10x10x0.4 m3 and weighed ~100 tons.
Uses
Together with the other potassium feldspars orthoclase is a common raw material for the manufacture of some glasses, some ceramics, such as porcelain, and as a constituent of scouring powder.
Some intergrowths of orthoclase and albite have an attractive pale lustre and are called moonstone when used in jewellery. Most moonstones are translucent and white, although grey and peach-colored varieties also occur. In gemology, their lustre is called adularescence and is typically described as creamy or silvery white with a "billowy" quality. It is the state gem of Florida.
The gemstone commonly called rainbow moonstone is more properly a colourless form of labradorite and can be distinguished from "true" moonstone by its greater transparency and play of colour, although their value and durability do not greatly differ.
3.Moonstone
Moonstone is a sodium potassium aluminium silicate, with the chemical formula (Na,K)AlSi3O8.
Etymology
Its name is derived from a visual effect, or sheen, caused by light reflecting internally in the moonstone from layer inclusion of different feldspars.
Geology
The most common moonstone is of the mineral adularia. The plagioclase feldspar oligoclase also produces moonstone specimens. Moonstone is feldspar with a pearly and opalescent luster. An alternate name is hecatolite.
Formation
Moonstone is composed of two feldspar species, orthoclase and albite. The two species are intermingled. Then, as the newly formed mineral cools, the intergrowth of orthoclase and albite separates into stacked, alternating layers. When light falls between these thin, flat layers, it scatters in many directions producing the phenomenon called adularescence.
Occurrence
Deposits of moonstone occur in: Australia, the Austrian Alps, Mexico, Madagascar, Myanmar, Norway, Poland, Sri Lanka, and the United States.
4.Amazonite
Amazonite (sometimes called "Amazon stone") is a green variety of microcline feldspar.
The name is taken from that of the Amazon River, from which certain green stones were formerly obtained, but it is doubtful whether green feldspar occurs in the Amazon area.
Amazonite is a mineral of limited occurrence. Formerly it was obtained almost exclusively from the area of Miass in the Ilmen mountains, 50 miles southwest of Chelyabinsk, Russia, where it occurs in granitic rocks. More recently, high-quality crystals have been obtained from Pike's Peak, Colorado, where it is found associated with smoky quartz, orthoclase, and albite in a coarse granite or pegmatite. Crystal Park, El Paso County, Colorado is a well-known locality for crystals of amazonite. Some other localities in the United States yield amazonite, and it is also found in pegmatite in Madagascar and in Brazil.
Because of its bright green colour when polished, amazonite is sometimes cut and used as a gemstone, although it is easily fractured.
For many years, the source of amazonite's color was a mystery. Naturally, many people assumed the color was due to copper because copper compounds often have blue and green colors. More recent studies suggest that the blue-green color results from small quantities of lead and water in the feldspar. (Hoffmeister and Rossman, 1985)
5.Labradorite
Labradorite ((Ca,Na)(Al,Si)4O8), a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It is usually defined as having "%An" (anorthite) between 50 and 70. The specific gravity ranges from 2.71 to 2.74. The streak is white, like most silicates. The refractive index ranges from 1.555 to 1.575. Twinning is common. As with all plagioclase members the crystal system is triclinic and three directions of cleavage are present two of which form nearly right angle prisms. It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.
Occurrence
The geological type area for labradorite is Paul's Island near the town of Nain in Labrador, Canada. It has also been reported in Norway and various other locations worldwide.
Labradorescence
It occurs in large crystal masses in anorthosite and shows a play of colors called labradorescence. The labradorescence, or schiller effect, is the result of light refracting within lamellar intergrowths resulting from phase exsolution on cooling in the Boggild miscibility gap, An48-An58.
