• Chemistry: KAlSi3O8, Potasium Aluminum Silicate.
  • Class: Silicates
  • Subclass: Tectosilicates
  • Group: Feldspars
  • Uses: mineral specimens and in the porcelain industry.
  • Specimens

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.


  • 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.
SANIDINE specimens:
(hover for more info)
SANIDINE specimen san-1
$ 40.00
Dims: 1-1/2" x 1-5/8" x 1-1/4"
Wt: 1.84 oz
Beaverdell, British Columbia, Canada
Finally, I get to see what a crystal of Sanidine looks like! To tell you the truth, it looks a lot like some orthoclase that I've seen! However, the surfaces, though comprised of the natural crystal faces, are rougher in texture. It has the classic form of members of its kind, complete with penetration twinning. There is a small amount of attached matrix that is weakly attached and can be easily removed if so desired. It has excellent crystal form, and would make a good addition to the collection of a "feldspar fanatic".
no photo
san-1 ($ 40.00)
Beaverdell, British Columbia, Canada
SANIDINE specimen san-2
$ 40.00
Dims: 1-1/2" x 1-5/8" x 1-1/4"
Wt: 1.84 oz
Beaverdell, British Columbia, Canada
Another specimen from what seems to be a prime Sanidine locality, this Canadian crystal has all the hallmark traits of the feldspar variation: excellent crystal form that has almost no damage, a grainy consistency, the lack of lamellar twinning, and a locality known for producing such material. There is a small amount of host rock that the crystal is attached to that contains small grains of what appears to be quartz, and other small, broken Sanidine crystals. The specimen may have been broken out and glued back into place, but it is difficult to be sure, as no glue is visible. There is, however, a small area on the matrix that is flattened and smooth where another crystal was probably broken off. A small portion of the matrix right next to the crystal is cracked and could be easily separated from the rest of the rock.
no photo
san-2 ($ 40.00)
Beaverdell, British Columbia, Canada
SANIDINE specimen san-3
$ 53.00
Dims: 2.4 x 1.4 x 1.0" (6.1 x 3.6 x 2.6 cm)
Wt: 4.4 oz. (125 g)
Beaverdell, British Columbia, Canada
This large Sanidine crystal is in good condition- it shows considerable damage, but this damage does not appear to be fresh- and has good monoclinic prismatic form, with slightly rounded edges and generally clean faces. Its color is a combination of pink and pale brown, and its luster is dull and waxy at best. There is no base or host rock present.
no photo
san-3 ($ 53.00)
Beaverdell, British Columbia, Canada
SANIDINE specimen san-4
$ 30.00
Dims: 2.5 x 2.4 x 1.8" (6.3 x 5.9 x 4.6 cm)
Wt: 4.8 oz. (136 g)
Beaverdell, British Columbia, Canada
Several Sanidine crystals are partly embedded in the gray host rock of this cabinet specimen. These crystals reach lengths of 1.3" (3.3 cm) and are in rather poor condition, as all are damaged and most are incomplete. Their monoclinic prismatic form is excellent where intact, however, and all have the pink-beige coloration, dull luster and almost pebbly surface texture that is common for the specie.
no photo
san-4 ($ 30.00)
Beaverdell, British Columbia, Canada
SANIDINE specimen san-5
$ 25.00
Dims: 1.5 x 1.1 x 0.7" (3.8 x 2.8 x 1.7 cm)
Wt: 1.4 oz. (39 g)
Beaverdell, British Columbia, Canada
This single Sanidine crystal is in fair condition, as its base end is broken and uneven. The rest of the crystal, however, is in excellent condition, and shows excellent monoclinic prismatic form. Its beige color and dull luster are common for the specie, and it is completely opaque. There is no host rock present, and the crystal is hot-glued to a small acrylic base.
no photo
san-5 ($ 25.00)
Beaverdell, British Columbia, Canada


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