Neosilicates: Others

Question 1

Staurolite

Answer 1

Hydrous (Magnesium iron aluminum) silicate
Formed in medium grade metamorphism especially schists
Monoclinic symmetry Al2SiO5 layers interleaved with layers of Fe-O tetrahedra

Question 2

Staurolite in thin sections using - 
Colour:
Pleochroism:
Form:
Relief:
Cleavage:
Birefringence:
Extinction:
Twinning:

Answer 2

Colour: Pale yellow
Pleochroism: distinct colorless to yellow-brown
Form: Subhedral
Relief: High
Cleavage: 010 but hard to distinguish
Birefringence: weak max 1st order yellow/red
Extinction: parallel
Twinning: no

Question 3

Staurolite in thin sections distinguishing features

Answer 3

Colour and strong pleochroism with low birefringence

Question 4

Sphene

Answer 4

Titanite monoclinic calcium titanium silicate
Common accessory mineral in plutonic igneous rocks but also Mafic metamorphic schists and gneisses
Monoclinic structure with TiO6 tetrahedra linked in zig-zag patterns with SiO4 tetrahedra

Question 5

Sphene in thin sections using - 
Colour:
Pleochroism:
Form:
Relief:
Cleavage:
Birefringence:
Extinction:
Twinning:

Answer 5

Colour: Colourless or pale/dark brown
Pleochroism: no
Form: euhedral lozenge 
Relief: high
Cleavage: non parallel parting
Birefringence: extreme but masked so view similar to ppl
Extinction: symmetrical 
Twinning: 100 common

Question 6

Sphene in thin sections distinguishing features

Answer 6

Diamond shape with cross parting

Question 7

Zircon

Answer 7

ZrSiO4
Rarely eroded so can be found quite old
Large quantities of uranium and thorium found inside are useful for dating rocks
Igneous but often found in sediments due to not being eroded
Tetragonal symmetry, alternating edge-sharing SiO4 tetrahedra and ZrO8 dodecahedra parallel to c axis

Question 8

Zircon in thin sections using - 
Colour:
Pleochroism:
Form:
Relief:
Cleavage:
Birefringence:
Extinction:
Twinning:

Answer 8

Colour: Colourless to pale brown 
Pleochroism: above
Form: small prismatic crystals
Relief: high
Cleavage: none
Birefringence: strong  0.06
Extinction: parallel
Twinning: rare

Question 9

Zircon in thin sections distinguishing features

Answer 9

Minute crystals in alkali-rich rock are often zircon. Higher relief and greater birefringence than apatite