ORTHOCLASE

General Formula:

KAlSi₃O₈

System: Monoclinic

Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Colour
Pleochroism

colourless

non pleochroic

Form

Common as anhedral to euhedral grains in igneous rocks.

Relief
RI

Low negative

n_alpha = 1.514 - 1.526

n_beta= 1.518 - 1.530

n_gamma = 1.521 - 1.533

Cleavage

Perfect parallel to {001}

Good parallel to {010}

Cleavages intersect at 90°

Birefringence
Interference Colours

0.005 – 0.008

No higher than first order white

Twinning

Range of twins may be present, eg. Carlsbad, Baveno, Mandebach.

Polysynthetic twins, as in plagioclase and microcline are not found.

Interference Figure
Optic Sign
2V

Biaxial

Negative

40 – ~70°

Optic Orientation

Optic plane is perpendicular to (010) and inclined between 5° and 13° to (001).

X^a = +5° to +13°, Y^c = + 21° to 13°, Z=b

Extinction is inclined to cleavage

Composition

Data

Alteration

Alters to clay minerals and sericite

Occurrence

Widespread in granite, granodiorite, pegmatite, syenite and related felsic rocks, emplaced at shallow depths.

Sanidine is more common in volcanic rocks and microcline more common in deep seated intrusives.

Common in contact and regional metamorphic rocks.

As a detrital grain in clastic sediments.

Distinguishing Features

Distinguished from sanidine based on 2V and from microcline by the lack of tartan twinning.