Glossary

A

acicular -
needle-like, long and thin.
adamantine -
brilliant and sparkling; a lustre such as that of diamond.
agglomerate -
a volcanically derived conglomerate or breccia.
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 -
an igneous rock texture of fine-grained groundmass with no phenocrysts.
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 clasts.

B

basement -
undifferentiated rock underlying strata of interest.
batholith -
large igneous intrusion that does not erupt, i.e. it is plutonic.
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.

C

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 sedimentary rock.
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.

D

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 calcite (CaCO3) but containing magnesium, chemical formula CaMg(CO3)2.
ductile -
deforms without breaking.

E

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 of cation.
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 -
an igneous rock texture applied to welded ignimbrites that have a planar fabric with flattened pumice clasts.
evaporite -
a mineral deposit formed by precipitation of minerals during the evaporation of mineral-rich fluids.

F

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 (SiO2).
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.

G

greasy -
a dull sheen, the appearance of being coated with an oily substance; a lustre such as that of talc.
groundmass -
the fine grained material of a rock in which larger crystals may be set; generally applied to igneous rocks (see matrix).

H

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 activity.

I

igneous -
produced by or associated with magma.
inert -
stable, does not change.
induration -
the process of forming rock by compaction and cementation (see diagenesis).
intrusive -
equivalent of plutonic, i.e. not erupted but cooled under ground.
ion -
an electrically charged atom or group of atoms.

L

laccolith -
lens or dome shaped intrusive igneous bodies.
lava -
molten rock erupted at the Earth's surface.
lithic -
made of rock.
lithology -
rock type.

M

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 (CaCO3) rich mudstone.
massive -
has no distinguishing crystal form.
matrix -
material filling spaces between larger clasts or crystals; generally applied to sedimentary rocks (see groundmass).
metallic -
has a shine like metal; a lustre such as that of pyrite.
metallurgical -
to do with the manufacture of metal.
metamorphosis -
change, alteration.
metasomatism -
the introduction or removal of elements to or from a rock during contact metamorphism.
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 (obsidian).
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 -
a conglomerate or breccia which has clasts of a single rock type.

N

nodule -
a spherical or rounded mass.
non-determinative -
of no use in identifying a mineral.

O

oligomictic -
see monomictic.
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 SiO4 tetrahedra (4-faced or triangular pyramids) with no shared oxygens (O) (also called nesosilicate), e.g. garnet, olivine.
oxide -
a mineral where oxygen (O) is combined with one or more metals, e.g. hematite, magnetite.

P

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 -
see polymictic.
phaneritic -
an 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 groundmass.
phosphorescent -
continues to glow when removed from an ultra-violet light source (see fluorescent).
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 -
a conglomerate or breccia which has clasts of different rock types.
polymorph -
one of two or more forms of a mineral with different crystal structures, e.g. graphite and diamond.
porphyritic -
an igneous rock texture of phenocrysts in a fine-grained groundmass.
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.

Q

quartzose -
sandstone consisting of more than 90% quartz clasts.

R

relict -
an original rock texture or structure preserved in a low grade metamorphic rock.
resinous -
has a resin-like shine resembling amber; a lustre such as that of some forms of calcite or garnet.
roots (of mountains) -
the very deeply buried parts of mountains that extend into the upper mantle.

S

schistosity -
a 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 SiO2; 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.

T

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.

U

upper mantle -
the uppermost part of the zone lying between the Earth's crust and the Earth's core.

V

vesicles -
gas filled cavities in magma or volcanic rock.
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 quartz.
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.

W

wacke -
a sandstone containing a significant amount of silt and / or clay.