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