Geologia Estructural Trucos

Siliciclastic sedimentary rocks
in thin section

1

Quartz SiO2
Properties
Light
Behavior

anisotropic-uniaxial

Extinction

possibly undulatory

Colour/
colourless/nonePleochroism

Interference Colours
(birefringence)

first-order white/gray (yellow possible especially
if the thin section is too thick)

Relief

low positive

Optic Sign

positive

Cleavagenone; displays conchoidal
fracture

2V

none

Crystal
Form

anhedral granular crystals which
are highly irregular

Alteration Products

none

Twinning

not visible in thin sectionQuartz is a tectosilicate that displays low relief, low birefringence and lacks
cleavage and twinning.

Dust line

Cement

2

Plagioclase
CaAl 2Si2O8(albite) - NaAlSi3O8(anorthite)Properties
Light Behavior

anisotropic-biaxial

Extinction

inclined

Colour/Pleochr
oism

colourless, may be clouded or none

Interference Colours
(birefringence)

first-order white/gray(yellow when more
calcic)

Relief

low negative or positive

Optic Sign

neg. or pos.

Cleavage

two intersect at 90°

2V

4 5°-102°

Crystal Form

crystals are tabular parallelto {010}, elongate parallel to
the c or a-axis, x -sections are rectangular, zoning is
common.

Alteration Products

sericite, saussurite

Twinning

albite, carlsbad , orpericline

•Plagioclase is a tectosilicate that has low relief, inclined extinction,
and displays polysynthetic twinning.
Anorthosite

Microcline KAlSi3 O8
Properties
Light Behavior

anisotropic-biaxialExtinction

inclined

Colour/Pleochro
ism

colourless, may be clouded/none

Interference Colours
(birefringence)

first-order
white

Relief

low negative

Optic Sign

negativeCleavage

two intersect at 90°

2V

6 5°-88°

Crystal Form

crystals are tabular parallel to {010}, elongate parallel to
c or a-axis

Alteration Products

sericite or
clay

Twinning...