thin_section_microscopy
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Raith, Raase, Reinhardt – January 2011
Guide to Thin Section Microscopy |
Conoscopy |
4.2.5.5 Practical hints for conoscopic observation
Mineral grains that are to be investigated conoscopically should meet specific requirements:
a.When using a strongly magnifying objective (MO = 40x, 50x, 63x or oil immersion 100x) with a large numerical aperture (>0.6), the mineral grain should cover the field of view completely.
b.The mineral grain must not be twinned within the observed frame, nor must it show alteration effects or exsolution phases.
c.The mineral grain should have a suitable orientation.
The procedure of obtaining the conoscopic interference figure is carried out as follows:
a.Move the mineral grain into the centre of the cross hairs, use a high-magnification objective (40x minimum) and focus.
b.Open all diaphragms below the stage.
c.Move the condenser into the uppermost position and put the auxiliary condenser into the light path.
d.Put the analyzer in. Take out the ocular; use a diopter if included with the microscope, observe the image in the ocular tube or use the Amici-Bertrand lens and observe the enlarged image through the ocular. In some microscopes, the Amici-Bertrand lens can be centred and focused separately, which is important for quantitative work.
e.If the mineral grain is too small to fully occupy the field of view, an upper diaphragm that is included in some microscopes can be used to reduce or eliminate the periphery around the grain. If there is no Amici-Bertrand lens with upper diaphragm, a sharp conoscopic interference figure may be obtained using a simple diopter in place of the ocular.
Minerals that are strongly coloured may cause problems for the determination of their optic sign if the first-order red plate is used. Therefore, the quartz wedge is recommended for compensation, which allows observing the direction of movement of the isochromes.
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Guide to Thin Section Microscopy |
Conoscopy |
Raith, Raase, Reinhardt – January 2011
Figure 4.2.5-10: Conoscopic interference figures of selected minerals, viewed in grain sections perpendicular to the optic axis or the acute bisectrix. Left-hand side: Interference figure without compensator; right-hand side: first-order red plate inserted.
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Raith, Raase, Reinhardt – January 2011
Guide to Thin Section Microscopy Scheme for mineral identification
5. Routine mineral determination
Observations in plane-polarized light mode
Mineral colour |
Light refraction |
Morphological |
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Colour tone and intensity, |
Relief, chagrin, |
characteristics |
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Grain form, twins, |
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Colour distribution |
Becke-Line: |
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Pleochroism |
Refractive index n |
fractures, cleavage, |
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Ch. 4.2.1 |
Ch. 4.2.2 |
zoning, inclusions, |
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decomposition, alteration |
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Ch. 3.1 – 3.4 |
Pleochroism, changing relief/chagrin = anisotropic non-cubic minerals
Observations in crossed polarizers mode
Mineral section remains black during rotation of stage
a. isotropic |
b. anisotropic: Section A optic axis |
no optic axis interference figure |
optic axis interference figure: |
Glasses; cubic minerals |
uniaxial, biaxial; Ch. 4.2.5 |
|
non-cubic minerals |
Mineral section changes from bright to dark during stage rotation = anisotropic
Interference colour |
Morphological features |
low, high |
Twins and intergrowths; |
normal, anomalous |
Subgrains, undulose extinction; |
Ch. 4.2.3 |
Zoning, unmixing; |
|
Brewster crosses in spherulites |
Sections of maximum |
Ch. 3.3, 3.4, 4.2.3 |
Sections with low |
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interference colour |
interference colour |
birefringence
Ch. 4.2.3
orthorhombic monoclinic
a. optically uniaxial, optic sign
Sections ~ A opt. axis
b. optically biaxial, optic sign
Sections ~ A acute bisectrix, or A optic axis
Ch. 4.2.5 |
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Extinction |
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straight |
hexagonal |
Sign of elongation: l (+), l (-) |
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symmetrical |
trigonal |
Ch. 4.2.4 |
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tetragonal |
Extinction angle H |
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inclined |
triclinic |
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Ch. 4.2.4 |
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