Name

Formula

Crystal System

Optics

Occurrences/Other

Microcline

KAlSi₃O₈

Triclinic

Biaxial negative; "Tartan" twinning is typical. Low, negative relief

Common in granites and high grade pelitic metamorphic rocks

Orthoclase

KAlSi₃O₈

Monoclinic

Biaxial negative; "Carlsbad" twinning is typical. Low, negative relief

Granites, rhyolites and other igneous rocks; higher Temperature polymorph

Sanidine

KAlSi₃O₈

Monoclinic

Biaxial negative; may show Carlsbad twins. Low negative relief

Exclusively in rhyolites and volcanic rocks - highest temperature polymorph.

Plagioclase Series:

All : (Ca,Na)(Al,Si)Si₂O₈

Triclinic

Biaxial Negative; "Albite Law" twinning is diagnostic

All varieties of igneous and metamorphic rocks

Anorthite (An)

CaAl₂Si₂O₈

Triclinic

As above

Mafic and ultramafic rocks

Albite (Ab)

NaAlSi₃O₈

Triclinic

As above

Granites and metamorphic rocks.

Oligoclase

(An 10-30)

Triclinic

As above

Granitic rocks

Andesine

(An 30-50)

Triclinic

As above

Intermediate rocks: Andesites and diorites

Labradorite

(An 50-70)

Triclinic

As above

Mafic rocks: can show iridescence in hand specimen

Bytownite

(An 70-90)

Triclinic

As above

Mafic rocks

Nepheline

NaAlSiO₄

Hexagonal

Uniaxial Negative: similar birefringence colors to Quartz

Common in alkaline intrusive and volcanic rocks.

Leucite

KAlSi₂O₆

Tetragonal

Uniaxial Positive; very low birefringence colors (pseudo-cubic)

Common in K-rich alkaline rocks.

Cancrinite

(Na,Ca,K)_(6-8) Al₆Si₆O₂₄ (CO₃,SO₄,Cl,OH)•H₂O

Hexagonal

Uniaxial Positive. Higher birefringence colors than Nepheline

Yellow in hand samples- commonly found with nepheline and sodalite