other names: Spat
mineral class: Framework silicates
chemical formula: (Ba, Ca, Na, K, NH4) (Al, B, Si)4O8
Chemical elements: Barium, calcium, sodium, potassium, nitrogen, hydrogen, aluminum, boron, silicon
Similar minerals: ?
colour: pure feldspar is colorless or white
shine: Glass gloss
crystal structure: monoclinic
mass density: approx. 2.6
magnetism: not magnetic
Mohs hardness: 6,5
stroke color: White
transparency: transparent to opaque
use: Porcelain production, gem stone
General information about feldspar:
Under the term feldspar are summarized several types of minerals, which are counted among the framework silicates. The name dates back to the 18th century and refers to their ability to easily split into fine spades when hitting them. All kinds of feldspar are not only their high cleavage but also their brittle to conchoid breakage and their striking glaze shine in common. Within the group, a distinction is made between the alkali feldspar or orthoclase, the soda-lime feldspar or plagioclase and the microlin, the group of so-called ternary feldspars, these three main types each being assigned to several subspecies.
All feldspar minerals are characterized as aluminosilicates by a three-dimensional network of tetrahedra, between which are arranged various positively charged cations. Their appearance is characterized by columnar or tabular crystals, which often occur in twinning. Depending on their location and chemical composition, they can either be transparent or opaque and show colors ranging from white to light pink, blue, green, red, orange or brown.
Feldspates are formed both in magma rock and in sedimentary rock and in metamorphic rock types. Depending on the melt, the chemical elements and the temperatures, different types of feldspar may develop in the rock.
In addition to quartz and kaolin, feldspar has played an important role in the production of porcelain for millennia. As early as the Stone Age, people in the region of today's China knew various techniques for making ceramics and glazing them at a high level. From the Chou dynasty, which began around 1122 BC, came artful porcelain containers, which were produced in the province of Chekiang and were already covered with glazes from feldspar. These items have a characteristic yellowish, gray to greenish hue and are considered the direct precursors of the legendary celadon ceramic, which is still a high priority in China. Feldspar still plays an important role as one of the most important ingredients of glazes for porcelain and ceramic tiles and is also used in its pure form for the production of dentures.
Locations and uses:
Also as gemstones are feldspar economically significant. The most beautiful specimens among the feldspars used in jewelry making include minerals belonging to the orthoclase group. The so-called moonstone with its characteristic milky and mostly light blue appearance and its shimmering in many colors surface is often honed as a cabochon. The most beautiful moonstones are found in the south of Sri Lanka, but also countries like Madagascar, Brazil, Burma, Australia, India and Tanzania produce worldwide coveted specimens of this dazzling orthoclase. In addition to the moonstone are still a second variety, namely known as sun stones plagioclase as gems particularly sought after. Already the Vikings and the natives of North America appreciated the crystals appearing in bright orange tones and attributed to them energetic potentials. The famous Oregon Sunstone has been officially recognized as a gemstone since the seventies of the last century. Ruby sun stones have been highly valued for several years. They come from Tibet and the Congo and are assigned to the group of Andesine.