Feldspar - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal System : Variable
    Chemistry : XZ4O8
    Rarity : Very common

Feldspars are the most abundant group of minerals in the Earth's crust, more than 55% of them. They are divided into two major groups : the alkaline feldspars (orthoclase, microcline, sanidine, anorthoclase) constituting a continuous series of a potassium pole at a sodium pole, and the calcosodic feldspars or plagioclases (albite, oligoclase, andesine, labradorite, bytownite and anorthite), also constituting a continuous series from a sodium pole to a calcium pole. There are other more rare feldspars such as celsian and hyalophane. Three origins are proposed for the term feldspar. "Feldtspat" comes from the Swedish feldt or fält and evokes the common presence of spath (feldspar) in fields plowed in granitic terrain. For others, this word comes from the German feld (field) and spat (crystal), because in decomposing the feldspar brings many enriching elements for the soil. Finally, some people think that this word comes from the German "fels" (rock).

Feldspars are essential minerals in almost all magmatic and metamorphic rocks ; they are less common in sedimentary rocks because they are easily altered in clay minerals. They are essential in the classification of magmatic rocks.

Alkaline feldspars :

Microcline and orthoclase are the alkaline feldspars common to plutonic rocks. Orthoclase appears in granites, granodiorites or syenites of medium depth, the microcline in pegmatites or in granites and syenites cooled slowly at great depth. The green amazonite is a variety of microcline, adular considered as a variety of orthoclase, can actually be also a microcline or sanidine... Sanidine is the alkali feldspar of volcanic rocks ; cooled quickly, this mineral has the most disordered structure of alkaline feldspars. Anorthoclase is an intermediate term between sanidine and high temperature albite.

Orthoclase from La Cabrera pluton, Madrid, Spain
Amazonite crystal from Pikes Peak, Colorado, USA
33.61 ct amazonite cabochon from Africa
Sanidine crystal from Chastreix, Puy-de-Dôme, France

Plagioclase feldspars :

Plagioclase feldspars offer a wide range of compositions but similar properties between sodium albite and calcium anorthite. They very often present polysynthetic twinning. This specificity makes it possible to easily recognize them with a polarizing microscope but also sometimes by magnifying glass, the twinning resulting in a fine and regular striation on the faces.

Plagioclase feldspars are very abundant and more widespread than potassic feldspars. They are mostly abundant in eruptive and metamorphic rocks, less in sedimentary rocks. The calcium terms crystallize at higher temperatures than the sodium terms, and the magmas formed early or at higher temperatures will mainly include calcium terms. The classification of magmatic rocks is largely based on the abundance and composition, calcium or sodium plagioclase.

1.29 ct faceted oligoclase from Brazil
13.00 ct sunstone from Tanzania
Labradorite slab from Madagascar
0.82 ct faceted andesine from Congo

Uses of feldspars :

Alkaline feldspars have more uses than plagioclase.

Orthoclase, microcline and more rarely sanidine are widely used in the porcelain and ceramics industry. They are also used as aluminous additives in the glass industry. Mixed with clay and potash, these feldspars are at the base of the glaze of porcelains and enamels. A minor part goes into making "soft" abrasives.

Ornamentation and gemstones constitute a notable additional use of these feldspars. Some varieties of transparent yellow sanidines of Madagascar are cut in gemstones. The amazonite, a blue-green variety of microcline is widely used as a fine stone and ornamental, and, under the name "moonstone" with opalescent bluish highlights, the shimmering varieties of anorthoclase, adularia, oligoclase and sometimes sanidine is used as a fine stone.

Among the plagioclases, albite and oligoclase are sometimes used in the ceramics and porcelain industry. Both have occasional rows of gemstones. Some specimens of oligoclase have a use in jewelery, in addition to the "moonstone" already mentioned, the best known is the "sunstone", a transparent oligoclase with red, brown or yellow inclusions. Finally, the labradorite-rich rocks are used as ornamental stones and in the object carving, their blue, yellow, purple hues are highly apreciated.

Hardness : Variable
Density : Variable
Fracture : Uneven to conchoidal

Streak : White

TP : Translucent to transparent
IR : Variable
Birefringence : Variable
Optical character : Variable
Pleochroism : Variable
Fluorescence : Variable

Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None