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A feldspar is a Minéral containing double Silicate of Aluminum, of Potassium, Sodium or Calcium. The feldspars are of the family of the Tectosilicate S. There exist many feldspars whose principal ones are the Orthose , potassic, the Albite , sodic, and the Anorthite , calcic. The mixture of these two last gives the series of plagioclases.

One can distinguish feldspars alkaline, often translucent, blanchâtres or pinks and rich in alkaline S (Na⁺, K⁺), of plagioclases which differ from the precedents by the presence of Calcium (Ca²⁺), and the absence of Potassium (K⁺).

The feldspars form the most important group of minerals of the Earth's crust. They are highly crystallized and have the property to form macles and to stripe glass. In combination with other minerals the potassic feldspars (or Alkaline S or Orthoclase S) compose of the rather acid rocks, the such Syénite S and the Granite S (Grenue S) or the Trachyte S and the Rhyolite S (Microlithique S). The feldspars plagioclases, compose as for them, concerning the grained rocks, the Diorite S, the Gabbro S, more rarely the Tonalite S and sometimes the Syénite S. When one finds some in the microlithic rocks, it is most of the time of Andésite S or Basalte S.

The potassic feldspar

The potassic feldspar, (KAlSi₃0₈), exists into three polymorphic which differs by scheduling Aluminum - Silicium in the tetrahedrons.

the Sanidine corresponds to the maximum degree of disorder: each tetrahedron contains on average 25% of Aluminum and 75% of Silicium. It is the polymorphic which intervenes at the highest temperature, its Groupe of space is C 2 m .
the Orthose corresponds to the partial order Aluminum - Silicium compatible with the monoclinical Symétrie : it is the polymorphic of intermediate temperature, its Groupe of space is the same one as that of sanidine: C 2 m . Its name comes from its faculty to be cleaved according to two plans ortho gonaux . It is present in the plutonic and metamorphic rocks, but is only seldom in the lava. Its hardness is precisely of 6 since it is one of the references of the scale of hardness Mohs. Its density lies between 2,56 and 2,58. The macle of Carlsbad is frequent.
the Microcline (see also Amazonite) is the polymorphic basic temperature: the order Aluminum - Silicium is not compatible any more with the monoclinical Symétrie and the microcline of it is triclinic makes. It can be differentiated from orthoclase only by one optical examination. The microcline watch often of the polysynthetic macle S (also named macles of the microcline).
the Anorthose - (K, Na) AlSi₃0₈ - is a sodic microcline .

Plagioclases

The plagioclases are calco-sodic feldspars. They form a continuous series between their two latest dates: the White feldspar (NaAlSi₃O₈) (see: Feldspar aventurine), sodic, and the anorthite (CaAl₂Si₂O₈), calcic.

Their Clivage (break) is oblique.

Table of plagioclases according to the proportion white feldspar/anorthite

Determination of plagioclases according to their color and them mâcle

With the naked eye, one often determines plagioclases according to their color, or of their Mâcle S. While the alkaline feldspars are in general rosâtres and present a bisynthetic mâcle known as of Karlsbad to two individuals, the feldspars plagioclases, them, often has colors pale green and presents contrary to the polysynthetic mâcles to many polarizing cleavages.

Determination of plagioclases under the polarizing microscope

If it is not possible to determine the type of mâcle, the various varieties of plagioclases are very similar and, in addition to the chemical analysis complexes and expensive, can be distinguished only by optical methods.

Plagioclases are characterized by the Méthode Michel Levy under the polarizing microscope by measuring the angle necessary so that the crystal blocks the passage of the light.

Measurement is done on plagioclases expenses, not zones.

Determination of the extinction angle of plagioclases:

To bring the main axis in North-South direction. The crystal must be gray uniform, with only the trace of the plans of macle.
Tourner platinizes it support of 45°. The crystal must be gray uniform. One should not see any more the trace of the plans of macle.
To return North-South.
To measure the extinction angle on a side, then other.
To make 5 measurements and to take largest.

Extinction angle according to the composition of plagioclases (approximate values)

Other feldspars

Among other feldspars, much rarer, it is necessary to mention:

the celsian , Mineral BaAl₂Si₂O₈, of the metamorphic Rock S rich person in Barium
the hyalophane , Mineral K_0.75Ba_0.25Al_1.75Si_2.25O₈, of the Rock S igneous and metamorphic
the buddingtonite , NH₄AlSi₃O₈·0,5H₂O, plagioclase deteriorated by action of Water X ammoniacal.

The deterioration of feldspars

The deterioration of feldspars provides various Argile S like the kaolin.

Use and source of feldspars

The feldspars are used out of ceramics, in particular for the manufacture of tilings. They also enter the composition of glasses used for packing (bottles). The principal supplying countries of European industry are the France, the Italy, the Turkey. ---- See also: Glossary of the minerals | Geology | List of the minerals

feldspar

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