Spectro 312B, Francis 2013
Lab 10: Reflected Light Microscopy
Sulfide minerals and many oxides are opaque to transmitted light and can only be optically studied
using reflected light. In addition, grains, seams, or inclusions whose dimensions are less than the
thickness of a standard thin section ( 30 microns) can not be well resolved in transmitted light, but
can be readily examined in reflected light. Furthermore, microprobe analysis requires an
examination of the material of interest under reflected light to insure that surface defects will not
degrade the analysis.
Because of the limitations of reflected light microscopy, it is a more qualitative art than transmitted
light microscopy. The process is essentially one of using the features of easily identifiable minerals
to constrain the identity of associated unknown minerals.
Today’s lab is designed to introduce you to the techniques of reflected light microscopy and
familiarize you with the appearance of some of the most common opaque minerals.
Optical Properties of Opaque Minerals:
 Reflectance and Colour
Reflectance is the measure of the ratio of the intensity of reflected light from a mineral’s surface
to the intensity of incident plane-polarized light ( = 546 nm). Although reflectance can be
quantitatively measured with suitable equipment, in general practice one qualitatively estimates
reflectance by comparing the unknown mineral to a known mineral.
Increasing reflectivity:
sphalerite (17%) < magnetite (21%) < galena (43%) < pyrite (54%) < gold (74%)
Colour is a more subtle feature in reflected light than in transmitted light, but can be very
diagnostic. For example, Fe-oxides are commonly grey, while many sulfides are distinctly
yellowish in colour. Sphalerite and galena are exceptions, however, being grey and greyishwhite respectively.
Note: Sulfide minerals tarnish easily, so it is best to buff them gently on a cloth with 0.3 micron
abrasive powder when first examining them.
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Spectro 312B, Francis 2013

Bireflectance and Reflection Pleochroism
As in transmitted light, isometric opaque minerals remain unchanged upon rotation of the
microscope stage. Anisotropic opaque minerals such as pyrrhotite, hematite and ilmenite,
however, may exhibit noticeable changes in reflectivity (bireflectance) and/or colour
(pleochroism) upon rotation of the microscope’s stage.
 Anisotropy
Isometric minerals appear either black under crossed polars, or remain dark grey upon rotation of
the stage. Anisotropic minerals may exhibit a noticeable variation in colour or brightness upon
rotation of the stage under crossed polars, exhibiting 4 positions of extinction and 4 positions of
maximum intensity or colour. These effects are often quite subtle and require careful
observation. It sometimes helps to rotate the analyzer of the microscope slightly from the 90o
crossed polar position to observe these features.
 Cleavage
Cleavage is often easily seen in polished surfaces in reflected light as dark lines and straight
sided pits, and can be characteristic of some minerals. For example, the polished surface of
galena characteristically displays distinctive triangular pits because of its three directions of 90 o
cleavage.
 Internal Reflections
Minerals that are not totally opaque sometimes display coloured internal reflections under
crossed polars when using bright illumination. Such internal reflections are characteristic of
minerals such as sphalerite and the ruby-silver sulfosalts (eg. proustite – pyrargyrite Ag3AsS3 Ag3SbS2). Internal reflections are also a good way of distinguishing silicate minerals in reflected
light.
 Polishing Hardness
The opaque minerals vary greatly in polishing hardness. Polishing hardness can be judged by the
quality of the polished surface (the hardest surfaces have the most mirror-like finishes) and can
be tested with a needle or by measuring relative polishing reliefs of adjacent grains using the
“Kalb line” test. The “Kalb line” is somewhat analogous to the “Becke line” in transmitted light.
When using the high power objective, and a partly closed diaphragm, lowering the stage will
cause the “Kalb line” to move from the grain boundary towards the softer of two adjacent
mineral grains.
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Spectro 312B, Francis 2013
Some Common Opaque Minerals: (listed in order of decreasing reflectance):
Formula
Reflectance
Colour
Anisotropy
Polishing
Hardness
Comments
Gold
Au
75
isotropic
2.5 - 3.0
Pyrite
FeS2
55
bright
yellow
pale
yellow
isotropic
6.0 - 6.5
FeAsS
52
white
strong
5.5 - 6.0
(Fe,Ni)9S8
47
isotropic
3.5 - 4.0
CuFeS2
44
weak
3.5 - 4.0
PbS
43
light
yellow
strong
yellow
grey
white
very bright &
soft
hard,
euhedral cubes
& triangles
euhedral
rhombs
exsolutions in
pyrrhotite
soft, yellow
isotropic
2.5
Pyrrhotite
Fe1-x S
34-40
pinkish
brown to
yellow
strong
4.0
Chalcocite
Cu2S
32
weak
2.5 - 3.0
ductile
Hematite
Fe2O3
25-30
strong
5.0 - 6.0
Cu5FeS4
22
isotropic
3.0
Magnetite
Fe3O4
21
light
grey
bluish
grey
pinkish
brown
brownish
grey
isotropic
5.5
Ilmenite
FeTiO3
17-20
pinkish
grey
strong
5.0 - 6.0
Sphalerite
(Zn,Fe)S
17
isotropic
3.5 - 4.0
Chromite
FeCr2O4
12
brownish
grey
dark
grey
isotropic
5.5
internal
reflections
tarnishes
violet/purple
lamellae of
anisotropic
ilmenite or
hematite
lamellae of
isotropic
magnetite
internal
reflections
internal
reflections
Mineral
Arsenopyrite
Pentlandite
Chalcopyrite
Galena
Bornite
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bright white,
cleavage
triangular pits
distinct
brownish tint
Spectro 312B, Francis 2013
Task: Identify Common Opaque Minerals:
Using first the naked eye and then the reflected light attachment for your microscope, describe and
identify the following minerals:
A. opaque mineral in one of sections: M-9 or BV-3
B. two yellow opaque minerals in one of sections: 7657, 7434, 7446, or RAD22
C. two brownish-yellow opaque minerals in one of sections: 8452, 8539, 8540, 8542, or 8543
D. grey-white and dark-grey opaque minerals one of sections: 6182, 6277, or 6455
E. weakly anisotropic and isotropic opaque minerals in one of samples: 886, 943 8401, or 2146
F. grey mineral in one of sections: RA-15, VII-1,VII-12, or VII-16a.
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