Thin Section Basics

Question 1

7 Properties

Answer 1

PPL:
Shape/Habit
Colour/Pleochroism
Cleavage
Relief

XPL:
Extinction
Birefringence
Twinning

Question 2

Rotation is used for (2)

Answer 2

Pleochroism

Extinction

Question 3

3 Crystal shape descriptors

Answer 3

Euhedral
Subhedral
Anhedral

Question 4

Euhedral

Answer 4

Well defined crystal shape with straight edges

Question 5

Subhedral

Answer 5

Some defined crystal shape with both straight and curved edges

Question 6

Anhedral

Answer 6

no well-defined crystal shape with rounded edges

Question 7

Habit

Answer 7

the tendency for specimens of a mineral to repeatedly grow into characteristic shapes (shape of the mineral)

Question 8

Acicular

Answer 8

Needle-like, e.g. Tourmaline

Question 9

Prismatic

Answer 9

length of the crystal is longer than its width it is called prismatic (eg. olivine)

Question 10

Tabular

Answer 10

Tabular crystals are flat and plate-like. They have lengths and widths that are much larger than their thickness.

Question 11

Granular

Answer 11

Granular is the habit of a crystalline aggregate composed of many rounded or equant anhedral crystals of approximately the same size.

Question 12

Equant

Answer 12

Said of a crystal having the same or nearly the same diameter in all directions

Question 13

Platy

Answer 13

platy are minerals which develop a crystal form consisting of thin, leaf-like layers. The mica group of minerals provides a good example

Question 14

Phenocryst

Answer 14

phenocryst is a crystal distinctly larger than the matrix in an igneous rock

Question 15

Porphyroblast

Answer 15

Porphyroblast is a large mineral crystal in a metamorphic rock that has grown within the finer-grained matrix.

Question 16

Porphyroclast

Answer 16

A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer-grained crystals

Question 17

Megacrysts

Answer 17

A large crystal in a fine-grained matrix where we are unsure where it is metamorphic or igneous

Question 18

Pleochroism

Answer 18

Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light.

Question 19

Cleavage

Answer 19

Cleavage means the separation by breakage of a mineral into fragments across planes of weakness

Question 20

twinkling

Answer 20

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.

Question 21

Relief

Answer 21

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

Question 22

Becke line test

Answer 22

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.

Question 23

Birefringence

Answer 23

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.

Question 24

Isotropic

Answer 24

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.

Question 25

Extinction

Answer 25

Extinction describes when cross-polarized light dims, as viewed through a thin section of a mineral in a petrographic microscope.

Question 26

Symmetrical Extiction

Answer 26

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).

Question 27

Twinning

Answer 27

Optically opposite extiction in the same mineral caused by the rotation or mirroring of one

Question 28

Polysynthetically twinned

Answer 28

Multiple twinning

Question 29

Zoning

Answer 29

Change between the core and outer rim of the mineral such as extinction or birefringence

Question 30

Speckled extinction indicates

Answer 30

Muscovite and Biotite micas are easily indentified in thin section because they almost always exhibit “speckled extinction” in cross polarized light

Question 31

Undulose extinction indicates

Answer 31

Quartz

Question 32

Psuedomorphs

Answer 32

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.

Question 33

Alteration

Answer 33

Mineral alteration refers to the various natural processes that alter a mineral’s chemical composition or crystallography

Question 34

Poikilitic

Answer 34

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.

Question 35

Oikocryst

Answer 35

The enclosing younger mineral in a poikilitic texture

Question 36

Chadacryst

Answer 36

The enclosed older mineral in a poikilitic texture

Question 37

Idioblastic

Answer 37

Euhedral for metamorphic minerals

Question 38

Granoblastic

Answer 38

A texture of roughly equidimensional grains formed by the mutual adjustment of grain boundaries in the solid state.

Question 39

Gneissic

Answer 39

A texture in which mineral aggregates are arranged in bands or linear streaks.

Question 40

Hornfelsic

Answer 40

A texture of fine-grained, equidimensional or randomly-oriented crystals, formed in response to heating without applied stress.

Question 41

Isomorphous

Answer 41

A crystal which has the same shape or structure as another mineral, but differs from it in composition.