Thin Section Basics Flashcards
7 Properties
PPL: Shape/Habit Colour/Pleochroism Cleavage Relief
XPL:
Extinction
Birefringence
Twinning
Rotation is used for (2)
Pleochroism
Extinction
3 Crystal shape descriptors
Euhedral
Subhedral
Anhedral
Euhedral
Well defined crystal shape with straight edges
Subhedral
Some defined crystal shape with both straight and curved edges
Anhedral
no well-defined crystal shape with rounded edges
Habit
the tendency for specimens of a mineral to repeatedly grow into characteristic shapes (shape of the mineral)
Acicular
Needle-like, e.g. Tourmaline
Prismatic
length of the crystal is longer than its width it is called prismatic (eg. olivine)
Tabular
Tabular crystals are flat and plate-like. They have lengths and widths that are much larger than their thickness.
Granular
Granular is the habit of a crystalline aggregate composed of many rounded or equant anhedral crystals of approximately the same size.
Equant
Said of a crystal having the same or nearly the same diameter in all directions
Platy
platy are minerals which develop a crystal form consisting of thin, leaf-like layers. The mica group of minerals provides a good example
Phenocryst
phenocryst is a crystal distinctly larger than the matrix in an igneous rock
Porphyroblast
Porphyroblast is a large mineral crystal in a metamorphic rock that has grown within the finer-grained matrix.
Porphyroclast
A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer-grained crystals
Megacrysts
A large crystal in a fine-grained matrix where we are unsure where it is metamorphic or igneous
Pleochroism
Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light.
Cleavage
Cleavage means the separation by breakage of a mineral into fragments across planes of weakness
twinkling
In mineral optics, the effect observed in plane-polarized light when a thin section of an anisotropic mineral with widely differing refractive indices is rotated rapidly on the stage. The ‘twinkling’ appearance is caused by the rapid changes in relief. Calcite is a well-known example.
Relief
Optical relief refers to the degree to which mineral grains stand out from the mounting medium, usually either oil with a known refractive index or Canada Balsam
Becke line test
The Becke line test is a technique in optical mineralogy that helps determine the relative refractive index of two materials. It is done by lowering the stage of the petrographic microscope and observing which direction the light appears to move.
Birefringence
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.
Isotropic
Isotropic minerals are minerals that have the same properties in all directions. This means light passes through them in the same way, with the same velocity, no matter what direction the light is travelling. There are few common isotropic minerals; the most likely ones to see in thin section are garnet and spinel.
Extinction
Extinction describes when cross-polarized light dims, as viewed through a thin section of a mineral in a petrographic microscope.
Symmetrical Extiction
the phenomenon which occurs when the extiction direction bisects the angles between two sets of cleavages (as seen in basal sections of pyroxenes and amphiboles).
Twinning
Optically opposite extiction in the same mineral caused by the rotation or mirroring of one
Polysynthetically twinned
Multiple twinning
Zoning
Change between the core and outer rim of the mineral such as extinction or birefringence
Speckled extinction indicates
Muscovite and Biotite micas are easily indentified in thin section because they almost always exhibit “speckled extinction” in cross polarized light
Undulose extinction indicates
Quartz
Psuedomorphs
Pseudomorph is a mineral or mineral compound that appears in an atypical form, resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another.
Alteration
Mineral alteration refers to the various natural processes that alter a mineral’s chemical composition or crystallography
Poikilitic
A texture visible in thin section where a mineral is found enclosed in another indicating the order of crystallisation as the enclosed mineral must have been solid whent he other formed so is older.
Oikocryst
The enclosing younger mineral in a poikilitic texture
Chadacryst
The enclosed older mineral in a poikilitic texture
Idioblastic
Euhedral for metamorphic minerals
Granoblastic
A texture of roughly equidimensional grains formed by the mutual adjustment of grain boundaries in the solid state.
Gneissic
A texture in which mineral aggregates are arranged in bands or linear streaks.
Hornfelsic
A texture of fine-grained, equidimensional or randomly-oriented crystals, formed in response to heating without applied stress.
Isomorphous
A crystal which has the same shape or structure as another mineral, but differs from it in composition.
