MINERALS DOWN THE MICROSCOPE
AUGITE {PYROXENE; (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6}:UNDER PPL:
Color:
Colorless
Pleochroism:
N.A
Refractive Index:
High
Brightness:
Medium
Relief:
High
Grain boundary:
Distinct
Cleavage:
2 sets
Grain shape and habit:
Subhedral and Prismatic
Alteration:
Sometimes present
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
2nd order green
Extinction:
Inclined extinction (45o)
Twinning:
Simple twin
Distinguishing Property: inclined extinction, higher
birefringence, 2nd order interference colours, low 2V, optically
positive in comparison with opx
Microcline {K(AlSi3O8)}:
UNDER PPL:
Color:
Colorless
Pleochroism:
N.A
Refractive Index:
Low
Brightness:
Low
Relief:
Low
Grain boundary:
Less distinct
Cleavage:
2 sets
Grain shape and habit:
Anhedral
Alteration:
Absent
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
1st order white to grey
Extinction:
Not studied
Twinning:
Tartan twin
Distinguishing Property: Commonly cross-hatched twin pattern
(tartan/gridiron) formed by intersecting albite and perthite twins.
It may be untwinned or have simple twins: commonly Carlsbad, Baveno
and Maneback twins. Exsolved albite may form lamellae, irregular
ribbons and blebs to form perthite.
Orthoclase (KAlSi3O8):
UNDER PPL:
Color:
Colorless
Pleochroism:
N.A
Refractive Index:
Low
Brightness:
Low
Relief:
Low
Grain boundary:
Indistinct
Cleavage:
2 sets
Grain shape and habit:
Subhedral
Alteration:
Absent
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
1st order white to grey
Extinction:
Roughly Parallel; 0-12o
Twinning:
Distinguishing Property Orthoclase is often confused with quartz
- try looking for cloudy patches on individual crystals in plain
polarized light which indicate weathering of the crystal. Also take
notice of the right angle cleavage
Hypersthene{(Mg, Fe)SiO3}
UNDER PPL:
Color:
Light pink to colorless
Pleochroism:
Yes
Refractive Index:
Medium
Brightness:
Low
Relief:
High
Grain boundary:
Distinct
Cleavage:
1 set prominent; 2 sets
Grain shape and habit:
Euhedral to subhedral
Alteration:
Present
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
Blue (2nd order)
Extinction:
Straight w.r.t cleavage
Twinning:
Absent
Distinguishing Property: Low birefringence, first order colors.
Parallel extinction in longitudinal sections, pale pink to green
pleochroism. Approximatly 90 cleavage planes. Thin irregular and
wavy lamellae common.
Enstatite
UNDER PPL:
Color:
Colorless
Pleochroism:
N.A
Refractive Index:
Medium to high
Brightness:
Low
Relief:
High
Grain boundary:
More or less distinct
Cleavage:
Grain shape and habit:
Alteration:
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
Extinction:
Twinning:
Distinguishing Property
Nepheline{Na3K(Al4Si4O16)}UNDER PPL:
Color:
Colorless
Pleochroism:
N.A
Refractive Index:
Low
Brightness:
Low
Relief:
Low
Grain boundary:
Indistinct
Cleavage:
1 set
Grain shape and habit:
Anhedral
Alteration:
Present
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
1st order grey
Extinction:
Straight w.r.t cleavage
Twinning:
Absent
Distinguishing Property: Resembles feldspars but nepheline is
uniaxial, lacks good cleavage and twinning, and has a low
birefringence. Nepheline is softer than quartz and differs in optic
sign.
Olivine{(MgFe)2SiO4}UNDER PPL:
Color:
Colorless
Pleochroism:
N.A
Refractive Index:
High
Brightness:
Medium to high
Relief:
High
Grain boundary:
Distinct
Cleavage:
Indistinct
Grain shape and habit:
Euhedral (6 sided)
Alteration:
Present and along fracture serpentinized
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
2nd order red
Extinction:
Straight w.r.t grain boundary
Twinning:
Absent
Distinguishing Property: Alteration, grain boundary and
interference color.
TremoliteUNDER PPL:
Color:
Pleochroism:
N.A
Refractive Index:
Brightness:
Relief:
Grain boundary:
Cleavage:
Grain shape and habit:
Alteration:
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
Extinction:
Twinning:
Distinguishing Property
}UNDER PPL:
Color:
Pleochroism:
N.A
Refractive Index:
Brightness:
Relief:
Grain boundary:
Cleavage:
Grain shape and habit:
Alteration:
UNDER XPL:
Isotropic/Anisotropic:
Anisotropic
Interference color:
Extinction:
Twinning:
Distinguishing Property