- acicular -
- needle-like, long and thin.
- adamantine -
- brilliant and sparkling; a lustre such as that of
- agglomerate -
- a volcanically derived
- aggregate -
- a mass of rock fragments and / or mineral grains.
- alkali feldspar -
- the term alkali means that the mineral contains the metallic elements potassium (K) and / or sodium (Na).
- aphanitic -
igneous rock texture of fine-grained groundmass with no
- arenaceous -
- sandy or sand-like, contains very little silt and / or clay.
- argillaceous -
- fine-grained or muddy, contains a significant amount of silt and / or clay.
- arkose -
sandstone consisting of more than 25% feldspar
- basement -
- undifferentiated rock underlying strata of interest.
- batholith -
igneous intrusion that does not erupt, i.e. it is
- biochemical -
- secreted by marine organisms such as algae and coral.
- bioclastic -
- formed from the shells of dead sea creatures.
- bipyramidal -
- shaped like two pyramids joined at their bases.
- botryoidal -
- the form of a mineral that occurs as joined spherical masses.
- bound -
- stuck together to form a larger mass.
- cataclastic -
- formed or deformed by shearing.
- cation -
- an atom or complex of atoms with a positive charge due to loss of electrons.
- clasts -
- fragments of pre-existing rocks or minerals that make up a
- clastic -
- made of clasts.
- conchoidal -
- smooth and curved; bowl-shaped.
- crenulation -
- small scale folding.
- cross-bedding -
- a sedimentary structure formed by wind or wave action.
- cryptocrystalline -
- individual crystals too fine to distinguish.
- crystallisation -
- formation of crystals from a liquid.
- deformation -
- geological process where the application of force changes the structure of existing formations, e.g. the folding
of flat-lying strata.
- degassing -
- loss of gas from lava due to loss of confining pressure.
- diagenesis -
- the changes that take place when turning sediment to rock.
- dike -
- a vertical sheet-like igneous intrusion that cuts through existing strata.
- disseminated -
- scattered throughout the rock.
- divalent -
- an atom which can combine with two other atoms or molecules.
- dolomite -
- a carbonate mineral similar to
3) but containing magnesium, chemical formula
- ductile -
- deforms without breaking.
- earthy -
- has the appearance of unglazed pottery.
- end-member -
- a mineral within a solid solution series whose available crystal lattice positions are filled with only one type
- equant -
- describes a crystal or
clast which has roughly the same dimensions along any axis.
- equigranular -
- consisting of minerals or
clasts of approximately the same size.
- extrusive -
- equivalent of
volcanic, i.e. erupted.
- eutaxitic -
igneous rock texture applied to welded ignimbrites that have a planar fabric with flattened
- evaporite -
- a mineral deposit formed by precipitation of minerals during the evaporation of mineral-rich fluids.
- fabric -
- the texture and structure of a rock.
- facies -
- a set of features that reflect the conditions under which a rock was formed or deposited.
- faulting -
- the breaking of rock through the application of stress.
- felsic -
- refers to generally light coloured minerals and rocks rich in silica (SiO
- fissile -
- breaks along planes of weakness.
- fluorescent -
- glows under ultra-violet light.
- folding -
- the bending of rock through the application of stress.
- foliation -
- a texture developed in
metamorphic rocks where exposure to high temperature and pressure causes alignment of minerals
forming distinct planes of weakness.
- greasy -
- a dull sheen, the appearance of being coated with an oily substance; a
lustre such as that of
- groundmass -
- the fine grained material of a rock in which larger crystals may be set; generally applied to
igneous rocks (see
- hand specimen -
- a piece of rock or mineral that can be picked up and easily transported.
- hackly -
- a fracture surface with sharp, jagged edges.
- hydrothermal -
- activity associated with the action of very hot water, associated with
- igneous -
- produced by or associated with
- inert -
- stable, does not change.
- induration -
- the process of forming rock by compaction and cementation (see
- intrusive -
- equivalent of
plutonic, i.e. not erupted but cooled under ground.
- ion -
- an electrically charged atom or group of atoms.
- laccolith -
- lens or dome shaped intrusive igneous bodies.
- lava -
- molten rock erupted at the Earth's surface.
- lithic -
- made of rock.
- lithology -
- rock type.
- mafic -
- refers to generally dark coloured minerals and rocks rich in iron (Fe) and magnesium (Mg).
- magma -
- molten rock beneath the Earth's surface.
- magma chamber -
- an area below the Earth's surface where magma is stored.
- marl -
- carbonate (CaCO
- massive -
- has no distinguishing crystal form.
- matrix -
- material filling spaces between larger
clasts or crystals; generally applied to
sedimentary rocks (see
- metallic -
- has a shine like metal; a
lustre such as that of
- metallurgical -
- to do with the manufacture of metal.
- metamorphosis -
- change, alteration.
- metasomatism -
- the introduction or removal of elements to or from a rock during
- micaceous -
- having a tabular, flaky form like that of mica.
- mineral -
- a naturally occurring, inorganic, solid, crystalline substance with a fixed structure and
a chemical composition which is either fixed or which may vary within certain defined limits.
- mineraloid -
- a substance without a fixed composition, e.g. volcanic glass (
- monoclinic -
- a crystal system (3-dimensional structure) with axes of 3 unequal lengths, two of which
intersect at an oblique angle (not 90°), and the other of which is perpendicular
(90°) to the plane formed by these.
- monomineralic -
- consisting of one mineral.
- monomictic -
breccia which has
clasts of a single rock type.
- nodule -
- a spherical or rounded mass.
- non-determinative -
- of no use in identifying a mineral.
- oligomictic -
- opaque -
- no light can pass through.
- ophiolite -
- oceanic crust which has been thrust in or on to a continental crust.
- ore -
- a rock or mineral from which economically important metals can be obtained.
- orthorhombic -
- a crystal system (3-dimensional structure) with axes of 3 unequal lengths, and which are mutually perpendicular
(90° to each other).
- orthosilicate -
- silicate mineral characterized by independent SiO
4 tetrahedra (4-faced or triangular pyramids) with no shared oxygens (O) (also called nesosilicate),
- oxide -
- a mineral where oxygen (O) is combined with one or more metals, e.g.
- pearly -
- a whitish iridescence resembling pearl; a
lustre such as that of some forms of mica.
- pegmatite -
- very coarse grained
igneous rock, usually granitic in composition, where individual crystals exceed 25mm in length.
- petrography -
- the systematic process of rock identification.
- petromictic -
- phaneritic -
igneous rock texture of large, easily visible, randomly distributed, interlocking crystals of several
minerals not aligned in any consistent direction.
- phenocryst -
- a large crystal in a finer-grained
- phosphorescent -
- continues to glow when removed from an ultra-violet light source (see
- pinacoid -
- an open crystal form consisting of two parallel faces. Pinacoids must combine with another crystal
form to make a crystal, and often form the top and bottom of prismatic crystals.
- plutonic -
- formed of
magma cooled beneath the Earth's surface.
- polymictic -
breccia which has
clasts of different rock types.
- polymorph -
- one of two or more forms of a mineral with different crystal structures, e.g.
- porphyritic -
igneous rock texture of
phenocrysts in a fine-grained
- porphyroblast -
- minerals that crystallise at a faster rate than
matrix minerals and so grow larger.
- pseudomorph -
- a mineral which has replaced another but retained the original mineral's shape.
- pyroclastic -
- literally means "fire-broken"; a term applied to
volcanic rocks or phenomena consisting of broken particles, generally produced
by explosive activity.
- quartzose -
- sandstone consisting of more than 90%
- relict -
- an original rock texture or structure preserved in a low grade
- resinous -
- has a resin-like shine resembling amber; a
lustre such as that of some forms of
- roots (of mountains) -
- the very deeply buried parts of mountains that extend into the upper mantle.
- schistosity -
foliation caused by the preferential alignment of tabular minerals.
- semiconductor -
- a material that is neither a good conductor of electricity nor a good insulator.
- silica -
- a polymorphic mineral with the chemical formula SiO
quartz is a common polymorph.
- silicate -
- mineral containing silica; silicates are the most abundant group of rock forming minerals.
- siliceous -
- contains or is made of silica.
- silky -
- has a sheen like that of silk.
- sill -
- a tabular igneous intrusion that parallels the strata into which it intrudes.
- solid solution series -
- describes a group of minerals that have the same crystal structure and same basic chemical composition.
Within a solid solution series
cations of similar size and charge can substitute for one another in the crystal lattice without
changing the crystal structure, and so form a series of minerals with the same basic physical properties
but with slight variations, e.g. colour.
- solute -
- a substance that is dissolved in another substance.
- sorting -
- a measure of the range of
clast sizes in a sediment: poorly sorted indicates a range of different sized clasts, well
sorted indicates that most clasts are the same size.
- sphenoid -
- a wedge-shaped four-faced solid form.
- strata -
- a group of rocks that have formed layers or beds.
- striations -
- a narrow groove or scratch.
- sub-metallic -
- intermediate between metallic and non-metallic.
- sub-parallel -
- refers to beds or features that are not exactly parallel due to variation in thickness.
- sulphide -
- a mineral in which sulphur (S) is combined with one or more metallic elements.
- tectonic -
- relating to deformation within the Earth's crust.
- texture -
- the shape, arrangement and distribution of minerals or grains /
clasts within a rock.
- thermoluminescent -
- glows when heated.
- translucent -
- light can pass through but is diffused so images cannot be seen clearly.
- transparent -
- light can pass through and images can be seen clearly.
- triboluminescent -
- glows when crushed, struck, scratched or, in some cases, rubbed.
- trivalent -
- an atom which can combine with three other atoms or molecules.
- twinning -
- a phenomenon occurring when a crystal consists of two or more parts, each part being the structural
reverse of the previous part.
- upper mantle -
- the uppermost part of the zone lying between the Earth's crust and the Earth's core.
- vesicles -
- gas filled cavities in
- vesicular -
- contains vesicles.
- viscosity -
- a measure of the resistance of a liquid to flow, e.g. honey has a higher viscosity than water.
- vitreous -
- glassy, shines like glass; a
lustre such as that of
- volcanic -
- formed of
magma erupted and cooled at the Earth's surface.
- volcanic arc -
- a generally curved line of volcanoes along the edge of a continent.
- wacke -
sandstone containing a significant amount of silt and / or clay.