Mineralogy

Mineralogy is the science that studies minerals

A mineral 

is a substance formed naturally, generally inorganic, exceptionally organic.

A given mineral is characterised by a chemical formula and a crystalline structure, that is respectively by the nature of the atoms that compose it and their arrangement in space. Mineralogy concentrates the various approaches to the study of minerals on these theoretical bases.

Several properties and methods allow the characterisation of a mineral. To study a given mineral, the mineralogist will use, among others:
• its crystalline structure in the solid state (most often studied using X-ray diffraction) 
• its chemical composition (often analysed by electron microprobe)  
• its mechanical properties: density or specific gravity, hardness, cleavage, fracture, breakage, touch  
• its optical properties: colour, streak, lustre, transparency, refractive index, interference analysis using X-rays  
• the bonds between atoms, which can be notably: covalent, ionic, metallic
• its chemical properties: photoluminescence, reactivity with acids, colouring under flame 
• its phase (solid, liquid or gaseous)  
• its solubility (in water and in acids)  
• its electrical and thermal properties. 

Mohs hardness scale

was invented in 1812 by the German mineralogist Friedrich Mohs to measure the hardness of minerals. Number 1 being the softest (talc) and number 10 the hardest (diamond).

The crystalline structure 

(structure of a crystal) is completely described by the parameters of its Bravais lattice, its space group and the position of atoms in the unit cell. These atoms repeat in space under the action of the symmetry operations of the space group and thus form the crystalline structure. This structure is a fundamental concept for many fields of science and technology.

A Bravais lattice 

is a lattice of nodes obtained by translation according to base vectors from a single node. Bravais lattices are classified into 14 types in three dimensions (5 types in two dimensions) and represent the periodicity of the crystalline structure. This is obtained from a minimal set of atoms occupying the asymmetric unit, repeated in space according to the operations of the crystal's space group. All crystalline materials have a periodicity corresponding to one of these lattices (but not quasicrystals). 

Cleavage

is the ability of certain minerals to break along flat surfaces in preferred directions when subjected to mechanical stress (a shock or continuous pressure). The existence and orientation of cleavage planes depend on the symmetry and crystalline structure. When fracture surfaces are irregular, it is called fracture. Cleavage and fracture are important criteria for mineral identification.
Sometimes there are separation planes that are not cleavage. These separations are not directly related to the crystal structure but are explained by variations in the lattice geometry caused by an accident: alteration, presence of twinning…
Cleavage is an important element that lapidaries take into account for gemstone cutting. Stones with easy cleavage can break, making them difficult to cut, the cleavage planes sometimes being very difficult to distinguish. Conversely, percussion cutting techniques take advantage of cleavage to shape a stone according to its natural crystalline form. Cleavage has also been used to split large crystals into smaller pieces.