SANIDINE (Potasium Aluminum Silicate)

SANIDINE


Sanidine is a polymorph of other minerals that share the same chemistry but have different crystal structures. If positive identification between these minerals can not be made by field methods then the specimen may simply be refered to as a potassium feldspar or K-spar. Plagioclase feldspars lack potassium, are light colored and are usually striated. The other k-spar minerals are orthoclase, microcline and anorthoclase.

The differences between these minerals are minor in hand samples. Microcline tends to be deeper colored and is the only one that can be, but is not always, a deep green (amazonite). Sanidine does not show the lamellar twinning that is common in microcline and is occassionally present as striations on cleavage surfaces.

Sanidine and anorthoclase usually have a flattened crystal habit. Other than that, environment of formation is the only other hand sample clue to distinguish orthoclase from sanidine. Sanidine (and anorthoclase) are a common constiuent in extrusive igneous rocks such as rhyolites, where the rock cooled quickly. Orthoclase is the main k-spar of granites and syenites that cooled somewhat more slowly, and microcline is the k-spar associated with granites, pegmatites, and syenites that cooled slowly. Optical properties and x-ray techniques are the only good ways to distinguish sanidine from orthoclase, microcline and anorthoclase.

Sanidine is the high temperature form of the k-spars. Above approximately 900 degrees C, sanidine is the stable structure. Between approximately 500 degrees C and 900 degrees C, orthoclase is the stable structure. And, at 400 degrees C or less, microcline is the stable structure for KAlSi3O8. The difference between the structures is only in the randomness of the aluminums and silicons. In microcline the ions are ordered and this produces the lower symmetry of triclinic (yes, higher more order produces lower symmetry, see discussion in symmetry). With higher temperatures the positions of the aluminums and silicons become more disordered and produce the monoclinic symmetry of orthoclase and finally sanidine.

Twinning is common in all feldspars and follow certain twin laws such as the Albite Law, the Pericline Law, the Carlsbad Law, the Manebach Law and the Baveno Law. In sanidine only the Carlsbad Law, the Manebach Law and the Baveno Law are seen. The Carlsbad Law twin produces what appears to be two intergrown crystals growing in opposite directions. Two different twin laws, the Manebach and Baveno laws, produce crystals with one prominant mirror plane and penetrant angles or notches into the crystal. Although twinning in general is common for sanidine, single crystals showing a perfect twin are rare and are often collected by twin fanciers.

Sanidine is an end member of a series of the alkali or K-feldspars whose series ranges from pure NaAlSi3 O8 to pure KAlSi3 O8. This series only exists at high temperatures with the mineral sanadine being the potassium, K, rich end member and albite being the sodium, Na, rich end member. Anorthoclase is the intermediate k-spar at about 10 to 36% sodium content.

PHYSICAL CHARACTERISTICS:

  • Color is off-white, yellow or pale shades of other colors.
  • Luster is vitreous to dull if weathered.
  • Transparency: crystals are transparent to translucent
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include tabular crystals. Crystals have a nearly rectangular cross-section with slightly slanted dome and pinacoid terminations. Also as rounded phenocrysts in volcanic rocks Twinning is common. (see above).
  • Cleavage is good in 2 directions forming nearly right angled prisms.
  • Fracture is conchoidal or uneven
  • Hardness is 6
  • Specific Gravity is approximately 2.56 - 2.53 (average)
  • Streak is white.
  • Associated Minerals are plagioclase feldspars, micas and other minerals found as phenocrysts in volcanic rocks.
  • Notable Occurrences Germany; Colorado and New Mexico, USA; Russia; Italy and others.
  • Best Field Indicators color, luster, lack of striations, cleavage, twinning if present and occurrence in volcanic rocks.
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