SANIDINE

General Formula:

(K,Na)AlSi₃O₈

System: Monoclinic

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

Colour
Pleochroism

colourless

non pleochroic

Form

Common as phenocrysts or tabular crystals

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

Carlsbad twins with a composition plane parallel to (010), dividing the crystal into two segments.

Interference Figure
Optic Sign
2V

Biaxial

Negative

2V_X = 0–40° (Low Sanidine)

2V_X = 0–47° (High Sanidine)

Optic Orientation

Two Orientations possible:

High Sanidine:

Optic plane is parallel to (010)

X^a = ~+5°, Y=b, Z^c = + 21°

Low Sanidine:

Optic plane is perpendicular to (010)

X^a = +5° to 9°, Y^c = + 21° to +17°, Z=b

Extinction is inclined to cleavage

Composition

Data

Alteration

Alters to clay minerals and sericite

Occurrence

Common K-feldspar in silicic volcanic rocks, eg. rhyolite, rhyodacite, phonolite and trachyte and in dykes of shallow intrusives. May occur in high temperature contact metamorphic rocks.

Distinguishing Features

Low Sanidine is distinguished from morthoclase by a smaller 2V_X angle.

High Sanidine is distinguished from low Sanidine and orthoclase by the orientation of the optic plane.