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Glossary of Geological Terminology from Quartz 'N' All Category: Earth and Environment > Geology Date & country: 15/11/2007, UK Words: 308

Magnetism
Magnetite is the only mineral which is strongly magnetic. It is that property that distinguishes it from chromite.

Malleable
Relates to the tenacity of a mineral. It means that a mineral slice will flatten when hit with a hammer. Copper is a good example.

Mammilated
( see habit) Round and mutually interfering masses, as in malachite; the protuberances are larger and more flattened than botryoidal.

Marine Transgression
Rise in sea-level, causing submersion of a land mass and the deposition of marine sediment. Conversely, marine regression causes deposition to cease and erosion to commence.

Marl
Mudrock composed of roughly equal amounts of clay and calcite

Massive
(see habit) Crystalline aggregates with no regular form.

Matrix
Fine grained material surrounding coarser grains, also known as groundmass.

Metabasite
Metamorphosed basic rock.

Metallic
A lustre produced by opaque minerals that absorb a lot of light.

Metamorphism
Metamorphic rocks are produced by the alteration of all types of rocks in the solid state under high temperatures and/or pressures.

Metapelite
Metamorphosed pelitic rock

Metasediment
metamorphosed sedimentary rock

Metasomatism
Where a host rock reacting with with hot fluids forms new minerals.

Mineral
Something natural made from inorganic substances. Its make up will include elements or substances, usually two or more, that cannot be broken down into other substances by chemical means. Some minerals like gold only have one element, these are known as 'native elements'.

Mineral Deposits
Unusual concentrations of minerals formed by special processes. When magma cools to form igneous rocks, minerals may become concentrated in particular areas.

Moh's Scale
A scale invented by Friedrich Moh in 1812 to define hardness of minerals on a scale of 1 - 10. 1 being the lowest, 10 being the highest. Each mineral can scratch another mineral with a lower hardness number, but can only be scratched by a mineral with a higher number. For more details, see the Fact File page.

Monoclinic
(see crystal systems) Crystals forming in the monoclinic system have 1 two fold axis of symmetry. Reference axes are of different lengths.

Native Elements
Native elements are minerals containing only one element within its composition.

Non-Metallic
A lustre shown by transparent or translucent minerals which can be subclassed :- Adamantine, Vitreous, Resinous, Pearly, Silky, Greasy and Earthy.

Odour
Some minerals can be identified by smell when either hit, rubbed or heated. Pyrite for example will give off a smell of burning sulphur when hit. Arsenic compounds when heated, give off a smell of garlic.

Opalescence
Milky blue or pearly appearance of the common opal.

Opalisation
Mottled play of colours of the precious opal.

Ophiolite
Suite of rocks comprising pillow lavas, dykes, grabbos and ultrabasic rocks. Thought to have formed from oceanic crust and now exposed at the surface.

Ophitic
(of igneous rocks) special type of poikilitic texture in which plagioclase is enclosed by pyroxene.

Orbicular Texture
Cencentric shells of a different texture and/or mineralogy from the parent rock.

Orogenic belt
Belt of rocks commonly characterised by deformation and folding, formed in a mountain building episode.

Orthorhombic
(see crystal systems) Crystals that form in the orthorhombic system have 3 perpendicular two fold axes of symmetry. 3 reference axes at right angles but differing lengths.

Pearly
Refers to lustre. The lustre of a pearl, caused by the reflection of light from a series of parallel surfaces within a crystal.

Pegmatite's
Frequently contain large crystals of minerals that would normally only grow in minute amounts. Pegmatite are formed in the last stages of crystallisation of the magma, when the liquid is rich in water and many uncommon elements. They commonly form veins and dykes around or within granite masses.

Pelagic
Of the deep sea. Usually clays or oozes composed of tiny silica or calcium carbonate skeletons

Pelitic
(Of rocks) Characterised by a high aluminium and low calcite content, especially clays and shales.

Perlitic
(Of glassy rocks) Containing irregular, spheroidal cracks which are formed by contraction during cooling.

Phencrysts
Large igneous crystals which contrast markedly in size with the surrounding crystals.

Phosphorescence
Minerals that glow in the dark for a time after being exposed to light give this effect.

Phytoplankton
small floating plants such as diatoms.

Piezoelectricity
Used to describe minerals that acquire an electric charge under stress. Quartz is a good example. It is this quality that allows quartz to be widely used in the electrical field.

Plug
Roughly vertical and cylindrical volcanic intrusion.

Pluton
Large coarse grained igneous intrusion.

Pneumatolitic Ore Deposits
Develop from the result of the late stages of cooling magma. Heated gasses containing volatile elements stream out into the adjacent country rock. The minerals eventually crystallise as pneumatolitic.

Pneumatolysis
Reaction of the hot gasses given off by a cooling magma with a surrounding country rock.

Poikilitic
(Of igneous rocks) Where large crystals of the host rock enclose small randomly orientated crystals of a different mineral.

Poikiloblastic
(Of metamorphic rocks) Crystals containing smaller ones with a porphyroblastic texture.

Polysynthetic
A repeated twin

Porphyritic
(Of igneous rocks) Comprising large crystals (phenocrysts) set in a finer grained or glassy groundmass.

Porphyroblastic
This texture describes large well shaped crystals surrounded by a finer grained matrix

Prismatic
A habit of of a mineral with faces that have grown better than other faces. For example, a mineral may show well developed prism faces, so it is much longer in one direction than the other two.

Pseudomorphism
Mineral formations with crystal shpaes which are not characteristic of the mineral. They develop because under certain conditions, crystals have dissolved away and the space left has been filled by another substance, during which process the crystal form has remained partially or wholly preserved.

Pyroclast
Fragment of volcanic rock scattered by explosive eruption

Pyroelectricity
This is used to describe the effect of minerals that become charged after heating. Tourmaline for example, is known to attract the ashes of a fire when heated and then repel them.

Quartz
(SiO2) The most common silica mineral.

Radiating
(see habit) Radial crystals or fibres, as in nodular pyrite.

Radioactivity
Some elements give off a Geiger counter reading of radioactivity. The most common mineral being pitchblende, a main ore of uranium.

Radiolaria
Single celled marine animals with a skeleton of silica

Re-entrant
Angles on a twinned crystal that point inwards.

Regional Metamorphism
The process whereby rocks have undergone a change, usually during mountain-forming episodes, where the rocks are subjected to elevated pressures and temperatures.

Relict Structure
Structure of the original rock which has persisted through metamorphism

Reniform
(see habit) Kidney shaped, as in hematite.

Resinous
The lustre of resin, as in amber and opal.

Ripple Marks
Small-scale undulations found in the bedding plains of sandstones and siltstones

Rock Forming Minerals
The minerals that make up most of the volume of rocks in Earth's crust. Most are silicates. The main ones are quartz, alkali, plagioclase feldspars, muscovite, biotite micas, pyroxenes. amphiboles and olivene. Calcite is also an important rock forming mineral in sedimentary rocks, along with dolomite.

Rocks
Are made up of lots of mineral grains and vary in quantity. Some may have one dominant mineral quantity but will always have a percentage, albeit a small one, of other minerals present.

Rudaceous
(Of sedimentary rocks) Coarse grained rocks specifically gravels conglomerates and breccias

Schiller - Schillerization
Strip-like light effects seen close to the surface of a mineral. Caused due to properties within the mineral involving reflection, interference and diffraction of light.

Schists
Medium-grained metamorphic rocks

Sectile
A term of tenacity. A sectile mineral can be cut with a knife, and the slice breaks up under a hammer.

Shields
Areas of crystalline basement rock associated with cratons

Silky
The lustre of silk. Usually confined to minerals that have a fibrous structure, such as satin spar.

Sill
Essentially horizontal, sheet-like igneous intrusion usually between two sedimentary beds, may be hundreds of metres thick.

Skarn
Limestones that have been altered to form a variety of minerals, including epidote, olivine, vesuvianite and ungrandite garnets, together with some ore minerals and boron-bearing minerals such as tourmaline.

Slates
Fine grained metamorphic rocks

Slump Bedding
Produced when an overlying bed slumps down into a weak underlying bed.

Solid Solution
Series of minerals where there is a complete gradation in composition between end members.

Specific Gravity
The ratio of the weight of a body to that of an equal volume of water. For more details see the Fact File page.

Spherulites
Spherical or sub-spherical bodies with a raidal arrangement of acicular crystals, often feldspar.

Spilite
Fine grained, basic igneous rocks richer in sodium and water than basalt and containing albite rather than plagioclase

Stock
Small batholith, with an area less than 10km2

Strata
see bedding

Streak
Streak is a good indicator in identifying minerals. Streak refers to minerals in their powdered form. The streak of a mineral will be consistent, regardless of colour variation in various specimens. A streak can be obtained by scraping with a penknife, or a piece of unglazed porcelain. For more details see the Fact File page.

Stufen
(Crystal Stufen) Mineral aggregate with several seperate individual crystals.

Sub-metallic
A lustre shown by semi-opaque oxides, such as hematite

Subhedral
Grains showing some crystal faces.

Sublimation
Process whereby a solid turns directly into a vapour on heating without passing through a liquid phase.

Subophitic
(Of igneous rocks) with feldspar crystals partly enclosed by pyroxene crystals of the same size.

Succession
Series of sedimentary rocks deposited in sequence

Suite
Group of rocks that are related to each other by a common origin or process.

Tabular
(see habit) Showing broad, flat surfaces, as in baryte.

Taste
Halite (rock salt) and sylvite can be told apart from their taste. Sylvite tastes a more bitter form than halite.

Tectonism
Large-scale movements and deformation of the Earth's crust.

Tenacity
Related to fracture. It refers to the way that minerals react to shock, crushing, cutting and bending. Can be described as Sectile, Malleable, Flexible, Elastic and Brittle.

Tetragonal
(see crystal systems) Crystals forming the tetragonal system have a single vertical axis of four fold symmetry. 3 reference axes all at right angles.

Transparency
An indicator used by mineralogists to determine whether a mineral is transparent, translucent or opaque but not a good field indicator to use on its own. Transparency depends on how thick the specimen is, and if there any inclusions present. Also dependant upon cleavage planes and internal flaws present.

Triclinic
(see crystal systems) Crystals forming within this system have no symmetry or just a centre of symmetry. 3 reference axes none of which are at right angles to the others.

Trigonal
(see crystal systems) Crystals forming under this system have a three fold axis of symmetry 3 horizontal reference axis, equal in length and at 120 degrees to each other.

Twinning
A twinned crystal is made up of two or four parts. One of its parts is in a reversed orientation to the adjacent part. Twins are recognised by the presence of re-entrant angles (angles that point inwards) Twinning can occur simply by contact across a flat surface, forming contact 'twins'. An example of this is rutile. In other cases, the twin cannot be divided into two separate halves, rather, the halves appear to have grown through each other. These are known as interpenetrant twins as seen in fluoirte. Twins may be single or repeated (polysynthetic) as in albite.

Ultrabasic
(of igneous rocks) containing less than 45% silica

Ultramafic
(of igneous rocks) Where dark coloured or mafic minerals make up more than 90% of the rock

Unconformity
Surface between sedimentary strata representing a time gap between periods of deposition

Uneven
Describes a type of mineral fracture. Very common and occurs where the surface of the mineral is characterised by minute elevations and depressions.

Vacuoles
Spaces left by materials in a magma that are easily vaporised, such as water and carbon dioxide.

Vein