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

Orthoclase 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 referred 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 sanidine, microcline and anorthoclase. Orthoclase is the more common of the k-spars.

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 blue-green color (amazonite). Orthoclase 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, enviroment of formation is the only other hand sample clue to distinguish orthoclase from sanidine or anorthoclase. Orthoclase is the main k-spar of granites and syenites that cooled moderately quickly. Sanidine and anorthoclase are common constituents in extrusive igneous rocks such as rhyolites, where the rock cooled quickly. Optical properties and x-ray techniques are the only sure ways to distinguish orthoclase from sanidine, microcline and anorthoclase.

Orthoclase forms at intermediate temperatures between the stability fields of sanidine and microcline. At 400 degrees C or less, microcline is the stable structure for KAlSi3O8. Between approximately 500 degrees C and 900 degrees C, orthoclase is the stable structure. And above approximately 900 degrees C, sanidine is the stable structure. The difference between the structures is only in the randomness of the aluminum and silicon atoms. In microcline the ions are ordered, and this produces the lower symmetry of triclinic (yes, 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 orthoclase, 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 orthoclase, single crystals showing a perfect twin are rare and are often collected by twin fanciers.


  • Color is off-white, yellow, or shades of red, orange to brown.
  • Luster is vitreous to dull if weathered.
  • Transparency crystals are usually opaque, may be translucent or rarely transparent.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include blocky or tabular crystals. Crystals have a nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations. Twinning is common. (see above). A psuedo-orthorhombic or psuedo-trigonal variety, found in alpine veins is called adularia, and forms more flattened tabular crystals.
  • Cleavage is good in 2 directions forming nearly right angled prisms.
  • Fracture is conchoidal or uneven
  • Hardness is 6
  • Specific Gravity is approximately 2.53 - 2.56 (average)
  • Streak is white.
  • Associated Minerals are quartz, plagioclase feldspars, micas, garnets, tourmalines and topaz.
  • Other Characteristics: some crystals may show opalescence and are called moonstone.
  • Notable Occurrences are many but these are a few of them: Salzburg, Austria; Cornwall, England and New York, Vermont, Maine and New Hampshire, USA.
  • Best Field Indicators color, lack of striations, cleavage, twinning if present and occurrence.

ORTHOCLASE specimens:
(hover for more info)
ORTHOCLASE specimen ort-1
$ 15.00
Dims: 3-1/8" x 2-1/4" x 2-3/4"
Wt: 8.9 oz
This almost perfect specimen of Orthoclase boasts one large penetration twin measuring 1-1/2 inches by 1 inch by 3/4 inch. It is exceptionally clean, with a tiny amount of surface damage and a large crack running straight across it. It appears to have been broken off and then reglued to the matrix, which looks to be a pegmatitic syenite that contains a small visible crystal and a smaller black, hexagonal section of an unknown crystal.
no photo
ort-1 ($ 15.00)
ORTHOCLASE specimen ort-2
$ 17.00
Dims: 3" x 1-3/4" x 2-1/2"
Wt: 6.8 oz
I haven't come across many Orthoclase crystals that looked as good as this one. It is a classic penetration twin measuring 1-5/8 inches by 1-1/8 inches by 1 inch, with very clean sides and minimal damage. Actually, most of the detectable damage is due to the crystal being broken out of the matrix and then glued back into position. The matrix seems to be made of a pegmatitic syenite that contains a few small visible crystals. It's a lovely crystal specimen.
no photo
ort-2 ($ 17.00)
ORTHOCLASE specimen ort-3
$ 25.00
Dims: 2-1/2" x 2" x 1-1/2"
Wt: 2.4 oz
3M Quarry, Little Rock, Arkansas, U.S.A.
How do explain this piece? There is a lot of different stuff in this specimen! There are several almost tabular prisms of Orthoclase on this specimen, all joined together through interpenetration. They are rather rough in appearance and some of them are damaged, but they are redeemed by the minerals that are attached to them. These include splendid crystals of analcime, tiny needles of sphene(shown in the close-up with an analcime "glob" right behind it) and books of biotite, and a pervasive crust of chlorite. All of these minerals occur in small but excellent crystals... All for a measly $25!
no photo
ort-3 ($ 25.00)
3M Quarry, Little Rock, Arkansas, U.S.A.
ORTHOCLASE specimen ort-4
$ 45.00
Dims: 8.7" x 4.9" x 3.9" (22.1 x 12.4 x 9.9 cm)
Wt: 4 lbs., 12.7 oz. (2.174 kg)
Valencia Mine, Guanajuato, Mexico
This very large Orthoclase specimen is an example of a variety known as Valencianite, named after the locality whence it was found. It consists of a large chunk of material that is made up of hundreds of intergrown crystals. Though none of these crystals are complete and separate, enough of each crystal is visible to determine their monoclinic form, though some appear to be almost rhombohedral. There are a few small areas of damage, but most are in excellent condition. The crystals' color is normally a pale cream, but most of this specimen has been heavily rust-stained. The crystals are basically opaque and have a rather bright, pearly luster even with the rust-staining. Oddly, they also show a subtle, colorful iridescence that may have something to do with the rust-staining, as that seems to be where it is most intense. Scattered on top of these Orthoclase crystals are hundreds and hundreds of tiny prismatic quartz crystals. These do not exceed 0.3" (8 mm), and though they would normally be colorless and clear, many of them are also rust-stained. Some of these are incredibly small (1 mm long or less), and a loupe is needed to really see them clearly. There is one portion of the specimen that is so heavily crowded with quartz crystals that none of the Orthoclase is visible. It is one of the oddest feldspar specimens that I have written about. There are a few patches of a dark host rock on the specimen's underside that have a definitely metamorphic appearance. A person who browses our site has made the suggestion that its locality name should not be used, as it is old and not conducive to accurate mineralogical nomenclature.
no photo
ort-4 ($ 45.00)
Valencia Mine, Guanajuato, Mexico
ORTHOCLASE specimen ort-5
$ 55.00
Dims: 1.9 x 1.5 x 1.0" (4.8 x 3.8 x 2.5 cm)
Wt: 1.60 oz. (45.5 g)
Organ Mountains, Dona Ana County, New Mexico, U.S.A.
This rather interesting specimen consists of a few weathered and intergrown Orthoclase crystals that are surrounded by and partly intergrown with dozens of smaller albite crystals. All but two of the Orthoclases are almost completely enveloped by the albites, and are extremely difficult to study. The two visible crystals have maximum dimensions of 1.1 x 0.5 x 0.4" (2.8 x 1.3 x 1.0 cm) and appear to be heavily weathered, though they show no visible damage. The larger one has fair but heavily rounded and worn monoclinic prismatic form, but the smaller crystal's form is defined well enough so that one can see that it is twinned. Both of these have a pale brown coloration and a dull pearly luster on their faces, and grow out of a base that is made up of larger, more heavily intergrown Orthoclase crystals. The albites that surround the Orthoclase crystals have a pale cream coloration and a brighter pearly luster. Though they are heavily intergrown, their triclinic prismatic form is still quite good, with well-defined edges and clean faces. Interestingly, all of them have grown in such a way that they are aligned in a certain direction!
no photo
ort-5 ($ 55.00)
Organ Mountains, Dona Ana County, New Mexico, U.S.A.
ORTHOCLASE specimen ort-6
$ 65.00
Dims: 1.6 x 1.8 x 1.4" (4.1 x 4.6 x 3.6 cm)
Wt: 2.13 oz. (60.3 g)
Organ Mountains, Dona Ana County, New Mexico, U.S.A.
This small hand specimen consists of a single Orthoclase crystal growing out of a formation of several heavily-intergrown albites. The Orthoclase is in very good condition, showing only a small amount of damage, and is heavily intergrown with the albite- it has visible dimensions of 0.6 x 0.6 x 0.5", but I think that the crystal is longer, and runs completely through the host. It shows moderately good form, with somewhat rounded edges and weathered but clean faces that possess a dull waxy luster. Though I am unsure, I believe that the Orthoclase crystal is twinned. It has a pale brown coloration that is slightly deeper than that of the pale cream coloration of the albite. The albite crystals are so heavily intergrown that it is difficult to effectively count them. They are in very good condition, showing almost no human-induced damage, and their form is also very good, even with their intergrowth. They have a pearly luster and are opaque, like the Orthoclase.
no photo
ort-6 ($ 65.00)
Organ Mountains, Dona Ana County, New Mexico, U.S.A.
ORTHOCLASE specimen ort-7
$ 48.00
Dims: 2.3 x 2.2 x 1.7" (5.8 x 5.6 x 4.3 cm)
Wt: 4.01 oz. (113.7 g)
Morro Redondo Mine, Coronel Murta, Minas Gerais, Brazil
This specimen is basically made up of a cluster of several Orthoclase crystals. These crystals are in excellent condition, as only a few of them are noticeably damaged. All have good monoclinic prismatic form, with moderately well-defined edges and patterned but clean faces that possess the standard waxy luster. At least 2 of the largest crystals are twinned, and possibly a third. They are so heavily intergrown that it is difficult to determine their sizes. All have the standard cream coloration of Orthoclase, and are dimly translucent around their edges. There is no host rock present.
no photo
ort-7 ($ 48.00)
Morro Redondo Mine, Coronel Murta, Minas Gerais, Brazil
ORTHOCLASE specimen ort-8
$ 90.00
Dims: 4.4 x 2.5 x 2.3" (11.2 x 6.4 x 5.8 cm)
Wt: 9.95 oz. (282.3 g)
Morro Redondo Mine, Coronel Murta, Minas Gerais, Brazil
At least 8 partly intergrown Orthoclase crystals, most of which are twinned, make up this hand specimen. These twins appear to be primarily contact twins, showing mirror images on each side of the twinning planes. These crystals range in size from a few millimeters along each axis to 1.5 x 1.5 x 0.9" (3.8 x 3.8 x 2.3 cm) and are in good condition, showing only a small amount of damage. Their monoclinic prismatic form is good even though their intense intergrowth interferes- all edges are well-defined and all faces are moderately clean and possess a dull waxy-to-pearly luster. Their color is the pale creamy-white that is standard for this mineral, and though a few may show dim translucence around their edges, they are generally opaque. A small amount
no photo
ort-8 ($ 90.00)
Morro Redondo Mine, Coronel Murta, Minas Gerais, Brazil
ORTHOCLASE specimen ort-9
$ 25.00
Dims:1.9x*1.1x0.8" (4.8x2.8x2.0 cm), 2.0x1.2x1.1" (5.1x3.0x2.8 cm)
Wt: 2.2oz. (63g)
Near Carlsbad, New Mexico
Two seperate pieces comprise this specimen. The first piece has a single prismatic crystal 0.6" (1.5cm) in length. This crystal is in excellent shape and displays excellent monoclinic form. It appears that it popped out of the matrix at one time and was repaired with a drop of glue. The repair job was good-it is not obvious. The second piece displays a perfect Carlsbad twin 0.5" (1.3cm) in length. Both of these pieces are on an orthoclase porphry matrix. They are undamaged.
no photo
ort-9 ($ 25.00)
Near Carlsbad, New Mexico
ORTHOCLASE specimen ort-10
$ 28.00
Dims: 1.3x0.8x0.5" (3.3x2.1x1.3cm)
Wt: 0.36 oz. (10.1g)
Epprechtstein, Fichtelgebirge, Germany
This is a very nice large thumbnail specimen of orthoclase, especially because it is a twinned crystal showing different terminations on the two halves of the twin. It is well formed, has an attached crystal of (I believe) albite, and a dusting of muscovite and other mineral crystals (some clear, some brown, some black). The original specimen labels are included - this is from a very old collection.
no photo
ort-10 ($ 28.00)
Epprechtstein, Fichtelgebirge, Germany
ORTHOCLASE specimen ort-11
$ 30.00
Dims: 5.97x1.98x1.62" (15.16x5.02x4.12cm)
Wt: 10.19oz (288.5g)
Dona Ana County, New Mexico, USA
This is a rather pleasing specimen of a mineral that is often boring. It is a cluster of stacked translucent white orthoclase crystals, and additional minerals line the cracks and crevices, especially between the crystals, providing a nice contrast. The crystals also have a pitted surface, which dulls their otherwise vitreous luster.
no photo
ort-11 ($ 30.00)
Dona Ana County, New Mexico, USA


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