• Volcan d'Islande
  • Améthyste
  • Orgue basaltique

The colour

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)

béryl vert variété émeraude

The coloured centres

Example of smoky quartz

The inclusions

biterminated hematoid quartz
Example milky quartz (fluid inclusions) or red (iron oxide inclusion)

The luster

Luster defines the way the mineral reflects light.

Metallic luster

pyrite en dodécaèdre
pyrite en dodécaèdre

Sphalerite diamond

Vitreous luster

Calcite optique

Calcite optique

Mica

Oily sheen

Quartz

Quartz

The resinous sheen

Soufre natif
Soufre natif

The shape

Initially, two categories of shape are distinguished: automorphic crystals and xenomorphic crystals.

Exemple cube de pyrite pour la forme automorphe
Exemple cube de pyrite pour la forme automorphe

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.

Exemple cube de pyrite
Exemple cube de pyrite

The tabular or shelf habitus

Exemple de la vanadinite
Exemple de la vanadinite

The Prismatic Habitus

Tourmaline noire variété schorl ou schorlite
Tourmaline noire variété schorl ou schorlite

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.
Example mica

The acicular or needle-like habitus

Exemple de la scolécite
Exemple de la scolécite

The filamentous or fibrous habitus

Example of asbestos

The aggregates

Aggregates indicate how the crystals fit together.

The radiating aggregates

 Exemple de l'aragonite en boule
Exemple de l’aragonite en boule

The cross-linked aggregates

Dendritic aggregates

The columnar aggregates

The fibrous aggregates

Stellar or hedgehog aggregates

Exemple de l'aragonite en boule
Exemple de l’aragonite en boule

Spheroidal aggregates

The botryoidal aggregates

Exemple d'un agrégat botryoïdal de prehnite
Exemple d’un agrégat botryoïdal de prehnite

The mamelon aggregates

The reniform aggregates

The lamellar aggregates

The feathery aggregates

The granular aggregates

The shape

The shapes of crystals can have several features:
Curved faces
black tourmaline schorl or schorlite variety

Striated faces
Example pyrite or quartz

Hardness

Find full details of the Mohs hardness scale.

The density

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.

Conchoidal fracture

Example of quartz, garnet, cordierite

Mineral with 1 cleavage plane

Example mica

Mineral with 2 cleavage planes

Amphibole pyroxene

Mineral with 3 cleavage planes

Optical calcite
Calcite

4 cleavage plane mineral

Musquiz fluorite
Fluorite

The trace

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.

The reaction to acids

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