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Mineralogy terms

Accessory

A mineral usually associated with a given rock but always in small quantities

Accidental

A mineral which can only occasionally be found in a given rock, and which usually does not appear at all

Acicular

A thin, rather elongated crystal

Aggregate

A group of crystals of the same or of different kinds

Cleavage

In minerals, it is the direction that a crystal usually breaks to produce a flat surface. There may be one or several cleavage directions

In rocks, the direction in which certain metamorphic rocks, particularly slate, split.

Crystal

Naturally occurring solid substance with flat surfaces

Earth's core

Central part of the earth, mostly made of iron

Earth's crust

Uppermost part of the earth. It varies from 30 to 70 kilometres beneath the continents, to only 6 to 8 kilometres beneath the oceans

Earth's mantle

The part of the earth lying between the core and the crust. It is some 2,900 kilometres thick, the upper part being mainly composed of olivine

Element

One of the simple substances of which all matter is made. i.e. Oxygen or Iron.

Fossil

Remains of (or an impression made by) a plant or animal found in rocks

Hardness

Hardness of a mineral is a measure of its resistance to being scratched. It can be given a number by reference to Moh's scale

Igneous rock

Rocks which were once molten (magma)

Intrusions

Individual masses of igneous rock formed when magma works its way in between rocks and solidifies before it reaches the surface

Lava

Molten rock with has poured out at the surface of the earth.

Magma

Molten Rock inside the earth. Solidifies to form igneous rock

Massive

Said of a shapeless lump or rock or mineral

Matrix

Rock in which fossils or crystals are held. Also the finer materials between the pebbles etc of a conglomerate or breccia

Metamorphic rock

Rocks which have been produced from other rocks by high temperatures and/or pressures.

Mohs' scale

Series of ten minerals arranged in order of increasing hardness against which the relative hardness of other minerals can be measured.

Nodule

A rounded lump of mineral, found in sedimentary rocks, which easily separates from the surrounding rock.

Rain prints

Marks on rock made by rain drops falling on it before it had finally hardened.

Sedimentary rock

Rock formed from sediment (sand, mud, animal remains, etc) on the surface of the earth

Streak

The colour of the powder of a mineral.

Texture

The relative size and shape of the grains of a rock

Translucent

A substance which allows light to pass through, but does not allow objects to be seen clearly through it.

Transparent

A substance which can be seen clearly through

Weathering

The breakdown of rocks at the surface of the earth by rain, hail, wind, frost etc.