Glossary

2V   In biaxial minerals, the acute angle between the optic axes; can be estimated by observing the greatest amount of separation between the isogyres as the stage is rotated.

acicular   Used to describe crystals that are needle-shaped.

aggregate  Grains that are clumped together into an irregular mass.

alteration   Chemical and/or mineralogical changes in a rock that occur due to weathering or reaction with hydrothermal fluids.

anhedral   Lacking well-developed crystal form.

anisotropic   Used to describe minerals with physical properties (such as index of refraction) that are not the same in all directions.

asbestiform   Used to describe fibrous minerals.

backscattered electron image   A form of scanning electron microscope analysis that records the electrons that bounce off the sample through high angles; reveals compositional variation.

basal section   A section through a crystal cut perpendicular to the c-axis.

Bertrand lens   A lens on an petrographic microscope that allows the viewer to observe interference figures.

biaxial   Minerals that contain two optic axes and three indices of refraction; includes the least symmetric minerals, those of orthorhombic, monoclinic, and triclinic crystal systems.

birefringence   Defined as n_s-n_f. Light traveling through a mineral is split into two rays, which travel at different speeds and have different indices of refraction: n_s is the index of refraction of the slow ray, n_f the index of the fast ray. Birefringence varies with the orientation of the mineral in the thin section, but a maximum value or a range of values for a certain mineral can be specified.

birds eye extinction    Found in micas such as biotite and muscovite. Different areas of the crystal become extinct as the microscope stage is rotated, forming a starry or mottled pattern somewhat similar to the grain seen in birds eye maple.

bladed   Used to describe crystals that are blade-shaped.

botryoidal  Used to describe minerals with a form similar to a bunch of grapes.

cathode luminescence  A type of scanning electron microscope analysis that records light emitted by minerals during electron bombardment.

cleavage   Crystallographic planes along which minerals tend to break due to bond strengths in the atomic structure.

cruciform  In the form of a cross.

detrital grains   Grains that are the disaggregated remnants of pre-existing rock.

devitrification   The crystallization of glass; occurs naturally.

element map   A type of electron microscope analysis that reveals the distribution of elements in the sample by recording the characteristic X-rays emitted by elements during electron bombardment.

embayed crystals    Crystals that have re-entrants or gouges, often due to having been partially resorbed, or dissolved, back into a magma.

energy dispersive analysis/spectra   A type of electron microscope analysis that reveals the chemical composition of a given mineral by recording the intensity of characteristic X-rays emitted from a mineral during electron bombardment.

euhedral   Possessing well-developed crystal form and faces.

exsolution   The unmixing of two phases of a solid solution; often occurs as temperature falls.

extinction   Occurs every 90° as the polars are rotated beneath a thin section on a petrographic microscope (or the microscope stage is rotated under crossed polarizers). The mineral becomes dark, or extinct, when one of the vibration directions of light passing through the mineral is parallel to the lower polarizer.

extrusive (or volcanic) rocks   Igneous rocks, such as lava flows and tuffs, that cooled relatively quickly at the surface of the earth; they generally lack large crystals.

foliated   Used to describe rocks or mineral aggregates that contain elongated minerals with a preferred orientation.

grain   In general, a single crystal of a mineral. (In sedimentary rocks, a grain is a particle that is between 0.03 mm and 2 mm in diameter.)

groundmass (or matrix)    The fine-grained material that encloses larger grains in sedimentary or igneous rocks.

gypsum plate   An accessory plate that can be inserted into a petrographic microscope while the Bertrand lens is engaged in order to determine if the mineral grain is optically positive or negative.

herringbone   A descriptive term indicating that crystals are aligned in parallel rows, with crystals in each row aligned in a mirror image of the adjacent row, like the textile fabric of the same name.

inclusion   In igneous rocks, a mineral grain or piece of rock that is surrounded by another mineral grain or rock.

index of refraction  Also known as refractive index, it expresses how much light is slowed down, and therefore bent, as it enters a substance. A high index of refraction means that light travels slowly in a mineral and is bent more. Index of refraction often varies as the chemical composition does, so a range is given.

interference colors  As light enters a mineral, it is split into two rays that vibrate at right angles to each other and interfere with each other. The colors produced depend on the thickness and birefringence of the mineral. Several orders of interference colors exist, generally decreasing in intensity as thickness and birefringence increase.

interference figures    Figures obtained with an optical microscope with the aid of the Bertrand lens that are used to determine if the mineral is uniaxial or biaxial and to find optic sign.

intrusive (or plutonic) rocks   Igneous rocks, such as granite or diorite, that cooled relatively slowly, beneath the surface of the earth. Characterized by phaneritic (visible to the naked eye), grains that often interlock.

isogyre  Areas of extinction in interference figures. The isogyres form a black cross in uniaxial interference figures, and may appear as one or two arcs in biaxial figures, depending on crystal orientation. When the stage is rotated, the biaxial isogyres will meet and separate.

isotropic   Minerals of the highly symmetric isometric crystal system, which have the same physical properties in all directions. Light travels at the same speed in all directions through isotropic minerals because the crystal lattice is identical in all directions. Only one index of refraction exists for the mineral. Isotropic minerals appear black at all times under crossed polarizers, regardless of stage rotation.

mafic   Refers to igneous rocks or magmas that are rich in minerals containing significant quantities of iron and magnesium; these rocks tend to contain lower concentrations of silica than silicic rocks, averaging around 45-50% silica.

massive   Lacking crystal form.

matrix   The finer-grained material that encloses larger grains in sedimentary or igneous rocks.

mud   Particles in sedimentary rocks that are less than 0.03 mm in diameter.

ooid   Carbonate grains formed of thin, concentric rings of calcite precipitated around a quartz or shell fragment nucleus; diameter is less than 2 mm.

optic axis   An imaginary line passing through a mineral. Light traveling through the mineral along this path behaves as if the mineral were isotropic, so the mineral will remain dark throughout polarizer (or stage) rotation if the axis is vertical.

optic sign   Using the gypsum plate with the Bertrand lens, optic sign can be determined. In uniaxial minerals, when one of the isogyres is oriented vertically, and the gypsum plate is inserted, if the color closest to the isogyres in the NE corner of the slide is yellow, the mineral is negative. If it is blue, the mineral is positive. In biaxial minerals, when the isogyres are oriented NE-SW, and the gypsum plate is inserted, if the color closest to the concave side of the NE isogyre is yellow, the mineral is negative. If it is blue, the mineral is positive.

parting  The splitting of a crystal along planes of weakness that are not cleavage planes.

peloid  Carbonate grain of microcrystalline calcite and aragonite that has no internal structure; usually is a fecal pellet of organisms that eat organic mud.

petrography  Field of geologic study dealing with the classification and description of rocks.

phenocryst  A larger crystal in an igneous rock that is clearly visible to the naked eye.

pisolitic  Texture of a rock formed primarily of pisoids (ooids greater than 2 mm in diameter).

pleochroism  Change of the color of certain minerals during stage rotation; seen in plane polarized light.

plutonic (or intrusive) rocks  Igneous rocks, such as granite or diorite, that cooled relatively slowly, beneath the surface. Characterized by phaneritic (visible to the naked eye), grains that often interlock.

polars (or polarizers) Filter in a petrographic microscope that allow only light vibrating in a single plane to pass through. Petrographic microscopes contain two polars, oriented at a right angle to each other. Sometimes the polarizers are called Nicols; some authors refer to the lower polarizing filter as the polarizer and the upper one as the analyzer.

porphyroblast  A large crystal formed during metamorphism; also known as a metacryst.

pseudohexagonal  Describes the roughly hexagonal form of crystals such as biotite that do not belong to the hexagonal class.

reaction rim  Rim formed around a mineral grain as it reacts with surrounding magma; forms when the mineral is not in chemical equilibrium with the magma.

resorption  Process in which a mineral is re-absorbed into the surrounding magma due to disequilibrium (perhaps caused by a change in pressure, temperature, or chemical composition of the magma).

silicic  Refers to igneous rocks or magmas that are rich in silica, containing about 65% or more SiO₂.

specularite  Variety of black or gray hematite that has a metallic luster and occurs as thin plates or masses of plates.

spherulite Subspherical aggregate of radial crystals in an otherwise glassy rock like obsidian.  Created by devitrification of glass.

subhedral  Crystals with both well-formed faces and poorly-formed faces.

trilling  Cyclic crystal twin that is made up of three crystals.

uniaxial  Minerals that contain one optic axis and two indices of refraction; includes minerals of tetragonal, trigonal, and hexagonal systems.

volcanic (or extrusive) rocks  Igneous rocks, such as lava flows and tuffs, that cooled relatively quickly at the surface. Characterized by glassy textures or phenocrysts scattered in a fine, microcrystalline matrix.

vesicular  Textural term used to describe volcanic rocks with numerous subspherical void spaces (or vesicles) formed by expansion of gas bubbles during cooling.

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