Term
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Definition
Colour: colourless to pinkish-brown
Transparency:moderate
Pleochroism:Normally none, but in coloured examples shades of pink- brown
Cleavage:2 good at 90° in basal sections, otherwise 1 good.
Relief:moderate-high
Extinction Angle:Inclined relative to single cleavage – maximum 45°
Interference colors:high 2nd order
Twinning:Common in basal sections, but not always present |
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Term
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Definition
Colour:brown-yellow
Transparency:good
Pleochroism:Straw-yellow to deep foxy-brown is what the older textbooks say.
Cleavage:1 perfect
Relief:low
Extinction Angle:straight
Interference colors:3rd order
Twinning:Never twinned
Alteration:Rare, but may be seen along cleavage traces |
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Term
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Definition
Colour:Variable. Pink, green brown
Transparency:moderate
Pleochroism:none
Cleavage:none
Relief:high
Extinction Angle:ISOTROPIC
Interference colors:NA
Twinning:none |
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Term
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Definition
Colour:Green or brown. Brown hornblende generally only occurs in volcanic rocks (andesite)
Transparency:moderate
Pleochroism:Shades of green─blue-green: dark-brown to orange-brown
Cleavage:2 good at ≈120° in basal sections, otherwise 1 good.
Relief:moderate-high
Extinction Angle:Inclined relative to single cleavage – maximum 18°
Interference colors:2nd order
Twinning:Common in basal sections, but not always present |
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Term
| Feldspar Group:Alkali feldspar “K-feldspars" |
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Definition
colourless
Transparency:good
Pleochroism:none
Cleavage:2 at 90°. However, these are rarely seen and are not diagnostic.
Relief:Moderate to low
Extinction Angle:depends on polymorph
Interference colors:1st order
Twinning:Albite polysynthetic, Carlsbad & Pericline may occur depending on polymorph |
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Term
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Definition
Colour: colorless
Transparency:good
Pleochroism:Sometimes developed, but weak; violet-blue
Cleavage:1 good, 1 perfect. Looks as if cleavage has been engraved on specimen
Relief:Very high
Extinction Angle:0-30°
Interference colors:1st order to low 2nd order
Twinning:Sometimes present, but only simple twinning |
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Term
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Definition
Colour:colorless
Transparency:good
Pleochroism:none
Cleavage:1 perfect
Relief:low
Extinction Angle:straight
Interference colors:3rd order
Twinning:Never twinned |
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Term
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Definition
Colour:colorless
Transparency:good
Pleochroism:Sometimes developed, but weak; pink-blue
Cleavage:2 at 89° commonly visible, one more prominent than the other
Relief:high
Extinction Angle:inclined
Interference colors:1st order
Twinning:none |
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Term
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Definition
Colour
colourless
Transparency
Very good
Pleochroism
none
Cleavage
none
Relief
low
Extinction Angle
Depends on morphology and symmetry
Interference colours
1st order
Twinning
never
Alteration
Very rare-never
Other
Easily strained such that extinction angle is varied within a single crystal. This gives “undulose” extinction |
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Term
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Definition
Colour
colourless
Transparency
good
Pleochrosim
Sometimes pale yellow-greenish brown
Cleavage
1 good. Not clearly visible if mineral occurs as fibrolite.
Relief
moderate
Extinction Angle
Straight along length of fibres.
Interference colours
2nd order
Twinning
none
Alteration
None
Other
-
Morphology
Often occurs as fibrolite but may be blocky. |
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Term
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Definition
Colour
colourless
Transparency
good
Pleochroism
none
Cleavage
none
Relief
moderate-high
Extinction Angle
Straight parallel to longest edges in well-formed crystals
Interference colours
high 2nd order
Twinning
never twinned
Alteration
Common – opaque minerals and serpentine
Other
Frequently exhibits curved cracks, often filled with opaque minerals in altered specimens
Morphology
Well-formed crystals are often six-sided with two long edges and two triangular terminations |
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