MICROCLINE

General Formula:

KAlSi3O8

System: Triclinic

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

Optical Properties

Colour
Pleochroism

colourless

non pleochroic

Form

Anhedral to euhedral in many igneous and metamorphic rocks

Relief
RI

Low negative

nalpha = 1.514 - 1.526

nbeta= 1.518 - 1.530

ngamma = 1.521 - 1.533

Cleavage

Perfect parallel to {001}

Good parallel to {010}

Cleavages intersect at 90°41’

Birefringence
Interference Colours

0.005 – 0.008

No higher than first order yellow

Twinning

Distinctive cross hatched or ‘tartan plaid’ twinning

Interference Figure
Optic Sign
2V

Biaxial

Negative

65 – 88°

Optic Orientation

Optic plane parallel to (001)

X^a ~18°, Y^c ~ 18°, Z^b ~18°

Extinction is inclined to cleavage

Composition

Data

Alteration

Alters to sericite and clay minerals

Occurrence

Found in granite, granodiorite, pegmatite, syenite and related plutonic rocks. Not normally found in volcanic rocks.

Common in regional metamorphic rocks of fairly high grade.

Common in immature clastic rocks.

Distinguishing Features

Tartan plaid twinning.

 


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