A mineral can have several colours (e.g. fluorite) but several minerals can have the same colour. A distinction is made between intrinsic colour and extrinsic colour.
The intrinsic colour
cut almandine garnet
Example red garnet coloured by divalent iron in its cubes
peridot or olivine cut into a drop
Olivine coloured green by divalent iron in its octahedra.
The extrinsic colour
The chromophore impurities
Beryl coloured green by traces of chromium (emerald)
The coloured centres
Example of smoky quartz
biterminated hematoid quartz
Example milky quartz (fluid inclusions) or red (iron oxide inclusion)
Luster defines the way the mineral reflects light.
The resinous sheen
Initially, two categories of shape are distinguished: automorphic crystals and xenomorphic crystals.
Each crystal system can give rise to several facies, i.e. several shapes from the original form. For example, by the play of truncations, the cubic form can give rise to forms derived up to the octahedron.
The habitus gives an indication of the relationship between the different dimensions of a crystal.
The isodiametric or equant habitus
The size ratio is close to 1. This habitus is systematically encountered for minerals of the cubic system.
The tabular or shelf habitus
The Prismatic Habitus
The barrel habitus
Example of corundum
The lamellar habitus
This habitus is close to the tablet habitus but the thickness of the crystal is much thinner.
The acicular or needle-like habitus
The filamentous or fibrous habitus
Example of asbestos
Aggregates indicate how the crystals fit together.
The radiating aggregates
The cross-linked aggregates
The columnar aggregates
The fibrous aggregates
Stellar or hedgehog aggregates
The botryoidal aggregates
The mamelon aggregates
The reniform aggregates
The lamellar aggregates
The feathery aggregates
The granular aggregates
The shapes of crystals can have several features:
black tourmaline schorl or schorlite variety
Example pyrite or quartz
Find full details of the Mohs hardness scale.
Density depends on the chemical composition of the mineral as well as the way its crystal structure is assembled (compactness).
Breaks and cleavages
Some minerals have the particularity of fracturing under the effect of a mechanical force according to preferential planes of breakage, which are called cleavage planes.
Example of quartz, garnet, cordierite
Mineral with 1 cleavage plane
Mineral with 2 cleavage planes
Mineral with 3 cleavage planes
4 cleavage plane mineral
The trace of a mineral is obtained by rubbing it on frosted porcelain. This therefore corresponds to the colour of the mineral’s powder. This only works for minerals with a hardness of less than 6 – 6.5.
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