EASC 3210: ORE MICROSCOPY

LECTURE 2, LECTURE 3

properties observed under crossed polars

• anisotropism

• internal reflections

covellite

blue, intensely pleochroic

gold

yellow, very high reflectance, very soft

chalcopyrite

yellow, very weak anisotrophy

bornite

(red) brown, weakly anisotropic

copper

(red) brown, high reflectance, very soft

reflectance

The percentage of light incident on the polished surface of a mineral that is reflected back through the microscope objective, to the observer

bireflectance

change in reflectance when rotated under ppl

reflection pleochroism

change in colour when rotated under ppl

cubic minerals generally do not show these properties

bireflectance and reflection pleochroism

strongly bireflectant mimnerals

graphite, molybdenite, covellite, stibnite

moderately bireflectant minerals

marcasite, hematite, pyrrhotite, cubanite, niccolite

weakly bireflectant minerals

arsenopyrite, ilmenite, enargite

BIREFLECTANCE and REFLECTION PLEOCHROISM are a function of…

the Crystallographic Orientation of the grain relative to the incident polarized light

anisotropism

• Cubic (Isometric) minerals remain uniform in appearance when the stage is rotated, although not necessarily completely dark

• exception is weak anomalous anisotropy in pyrite

The maxima and minima of these anisotropic effects will each occur…

four times in a 360º rotation, offset 45º from each other

false anisotropy can be introduced by …

fine parallel scratches

marcasite

has distinct deep blue/green/yellow anisotropic colours

internal reflections

• generated by light reflected back to the surface from subsurface cracks or flaws in the grains of a non-opaque mineral

• view under crossed polars with brightness increased

• ex. quartz

sphalerite

shows yellow, brown, green or red internal reflections

internal reflections useful in identifying…

quartz, sphalerite, cassiterite, hematite, wolframite, and chromite

hardness

• harder minerals stand out (positive relief) from softer minerals

• due to polishing relief

kalb light line test

1. Focus on a clear boundary line between two mineral grains

2. Lower the stage to defocus the sample

3. Observe a “Becke-like” line of light, which will move into the softer mineral

which mineral is harder?

Ttr

pyrite

very hard mineral that retains diagnostic deep scratches from initial prep

gold, silver, molybdenite, graphite

very soft minerals that almost always retained a scratched or dimpled appearance after polishing

framboidal/spheroidal

common in shale-hosted pyrite, is a product of bacterial activity that precipitates iron sulphides

galena

has triangular pitting due to cubic cleavage that causes plucking features during polishing

marcasite

irregular longitudinal fractures

molybdenite

basal cleavage/lath shape

chalcopyrite disease

• sphalerite from VMS (or others) with small inclusions of chalcopyrite

• oriented along crystal planes

• are a later hydrothermal replacement feature

paragenesis

• the time sequence of formation of mineral phases

• shown from the textural relationships of minerals in thin or polished sections