Glossary of Common Thin Section Terms

Alignment

The arrangement of mineral grains, crystals, or structural features in a common direction.

Alteration

The chemical or mineralogical change of an original mineral into one or more new minerals, usually caused by fluids, weathering, or changing conditions.

Anhedral

A crystal that lacks well-developed crystal faces and has an irregular outline.

Aphanitic

A fine-grained texture in which individual crystals cannot be distinguished easily with the naked eye.

Bedding

Layering in sedimentary rocks caused by changes in sediment deposition, composition, grain size, or color.

Biaxial

An anisotropic mineral with two optic axes. belongs to the orthorhombic, monoclinic, or triclinic systems.

Birefringence

The difference between the refractive indices of light traveling in different directions through an anisotropic mineral. It produces interference colors under crossed polars.

Blocky

A crystal or grain shape that is roughly equidimensional and block-like, rather than elongated or needle-shaped.

Cataclasis

The breaking, crushing, and fracturing of rocks or minerals during deformation, commonly associated with faulting.

Cement

Material precipitated between sedimentary grains that binds the grains together into a solid rock.

Clast

A fragment of preexisting rock, mineral, or other material that forms part of a sedimentary rock.

Cleavage

The tendency of a mineral to break along specific crystallographic planes of weakness.

Coarse grain size

Relatively large mineral grains. In many introductory classifications, coarse grains are larger than approximately 5 mm, although exact limits can vary.

Crystallinity

The degree to which a rock consists of crystalline material rather than glass.

Dimensions

The measurable size of a grain or crystal, commonly described using length, width, and thickness.

Euhedral

A crystal with well-developed faces that reflect its characteristic crystal form.

Exsolution (lamellae)

The separation of a solid solution into two distinct mineral compositions during cooling or changing conditions. Lamellae are thin, commonly parallel bands of the exsolved mineral.

Extinction

The condition in which an anisotropic mineral becomes dark under crossed polarized light as the microscope stage is rotated.

Extinction angle

The angle between a mineral's cleavage, elongation direction, or another reference feature and the position where it becomes extinct.

Fine grain size

Relatively small mineral grains. A common introductory classification defines fine grains as less than approximately 1 mm, although limits may vary.

Flow bands

Bands or layers produced when magma or lava flows, aligning crystals, glass, or compositional material in the direction of flow.

Frosted grains

Grains with a rough, dull, or frosted-looking surface, commonly caused by abrasion, weathering, or surface alteration.

Grain size

The diameter or dimensions of individual mineral grains or rock fragments.

Grain size distribution

The range and relative proportions of different grain sizes in a rock or sediment.

Bimodal grain size distribution

A grain-size distribution with two dominant grain-size populations, such as mostly large grains and mostly small grains.

Large grains

Grains that are relatively large compared with the other grains in the sample.

Small grains

Grains that are relatively small compared with the other grains in the sample.

Average grain size

The representative or typical grain size of a sample, often estimated from several grains rather than one grain.

Granular

A texture consisting of relatively equidimensional grains that resemble small granules.

Hyaline

Glassy or noncrystalline material; a ______ rock or portion of a rock consists primarily of glass.

Interference color

Colors produced when light rays traveling through an anisotropic mineral interfere with one another under crossed polarized light.

Interstitial grains

Grains or crystals that occupy spaces between larger grains or crystals.

Irregular grains

Grains with uneven, nonuniform, or poorly defined outlines.

Isotropic grains

Grains with the same optical properties in every direction. They remain dark under crossed polars during stage rotation.

Lath

A long, narrow, commonly rectangular or blade-like crystal shape, frequently seen in feldspar.

Lithic fragments

Rock fragments composed of pieces of preexisting rock rather than individual mineral grains.

Matrix

Fine-grained material surrounding and filling the spaces between larger grains or clasts.

Medium grain size

An intermediate grain size between fine and coarse. The exact size range depends on the classification system being used.

Mineral percentage

The proportion of a rock made up by a particular mineral, commonly expressed as a percentage of the rock's volume.

Mosaic

A texture consisting of interlocking grains or crystals that form a patchwork-like pattern.

Myrmekite

A microscopic intergrowth of vermicular quartz within plagioclase, commonly associated with alteration or replacement involving feldspar.

Oolite

A small, rounded sedimentary grain, usually composed of concentric layers of carbonate material around a central nucleus.

Opaque mineral

A mineral that does not transmit visible light through a standard thin section and appears black in transmitted light.

Pegmatitic

A very coarse-grained igneous texture, commonly containing exceptionally large crystals.

Phaneritic

A coarse-grained igneous texture in which individual crystals are large enough to be seen with the naked eye.

Phenocryst

A relatively large crystal within a finer-grained or glassy igneous groundmass.

Pleochroism

The change in a mineral's color as it is rotated in plane-polarized light due to different light absorption in different crystallographic directions.

Porosity

The percentage or proportion of empty space, or pores, within a rock or sediment.

Porphyroblast

A relatively large metamorphic crystal that grows within a finer-grained matrix during metamorphism.

Relief

How strongly a mineral's grain boundaries and surface appear to stand out in plane-polarized light because of differences in refractive index between the mineral and its surroundings.

Rounded grains

Grains whose edges and corners have been smoothed by abrasion, transport, or weathering.

Sericitic

Describes alteration involving the formation of fine-grained sericite, usually a fine white mica, commonly replacing feldspar.

Shape

The overall geometric outline or form of a grain or crystal, such as rounded, blocky, elongated, or irregular.

Sign (optical sign)

Indicates whether an anisotropic mineral is optically positive or negative based on the relationship between its refractive indices and optic axes.

Sorting

The degree to which sediment grains are similar in size. Well-sorted sediment has a narrow grain-size range; poorly sorted sediment has a wide range.

Subhedral

A crystal that has some well-developed crystal faces but also has irregular or undeveloped boundaries.

2V

The angle between the two optic axes in a biaxial mineral. It is used to describe and identify the mineral's optical properties.

Twinning

A symmetrical intergrowth of two or more crystals of the same mineral with a specific crystallographic relationship.

Uniaxial

An anisotropic mineral with one optic axis. Uniaxial minerals belong to the tetragonal, hexagonal, or trigonal systems.

Wentworth scale

A grain-size classification system used to categorize sediment particles, ranging from clay and silt to sand, gravel, and larger particles.

Vesicles

Small holes or cavities in volcanic rock formed when gas bubbles become trapped in cooling lava.

Xenolith

A fragment of preexisting rock enclosed within an igneous rock.

Zoned crystals

Crystals that contain compositional or optical differences from the core to the rim, usually caused by changing conditions during crystal growth.