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,
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,
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
Fracture is conchoidal or uneven
Hardness is 6
Specific Gravity is approximately 2.53 - 2.56 (average)
Streak is white.
Associated Minerals are quartz,
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,
Best Field Indicators color, lack of striations, cleavage, twinning
if present and occurrence.