A olivine is a Minéral of the family of the Nésosilicate S and a Semiprecious stone used in Joaillerie under the name of Péridot. It crystallizes in the crystalline Système orthorhombic. Sections often Automorphe S in the volcanic rocks, rather xenomorphic S in the grained rocks. Have a strong relief with the observation with the polarizing Microscope, as well as a strong birefringence, dyed sharp of 2nd with the 3rd order under the polarizing microscope in analyzed light.

The denomination of this mineral was given to him by the German mineralogist Abraham Gottlob Werner in 1790 in reference to its color green olive.

Composition

The general formula of olivines is XYTO₄, where:

• tetrahedral cation (If)

• bivalent cation in the octahedral sites m2

• bivalent cation in the octahedral sites M1

The composition of natural olivines is included/understood in a tetrahedron having by tops the phases:

• Forsterite (Fo): Mg₂SiO₄

• Fayalite (Fa): Fe₂SiO₄
• Tephroite: Mn₂SiO₄
• Bredigite: Ca₂SiO₄

The calcic member is constituent important metallurgical Ciment Portland cement and slags. He also returns in the composition of agricultural fertilizers. He gives several polymorphic and in nature he seldom presents himself with structure olivine, although this one is the basic phase temperature.

In the tetrahedron of composition one observes two gaps of miscibility, which correspond to the phases:

• CaMgSiO₄ Monticellite
• CaFeSiO₄ Kirchsteinite

By analogy, one gives also a name particular to the two intermediate components even if they are not intermediate phases:

• MnMgSiO₄ Picrotephroite

• MnFeSiO₄ Knebelite.

Along the axis Fo - F, where one finds the olivines most important, which are classified in the following manner:

• Fo100-Fo90: Forsterite

• Fo90-Fo70: Chrysolite
• Fo70-Fo50: Hyalosiderite
• Fo50-Fo30: Hortonolite
• Fo30-Fo10: Ferrohortonolite
• Fo10-Fo0: Fayalite

Olivines containing quantities concerned with Zinc and content minor of Nickel and Chrome are also known.

Paragenesis

In the earth's crust, the members rich in Mg are important components of the mafic and ultramafic igneous rocks ; they are also in metamorphized dolomitic limestones thermically. The members rich in Fe are minor phases of the alkaline igneous rocks and metamorphized iron-bearing sediments. With high percentages of magnesium the forsterite coexists with periclase (MgO). On the other hand, with high percentages of SiO₂ the forsterite is transformed into Enstatite.

Olivine is the mineral dominating of the Péridotite S, rocks constituting the coat. A dunite is a rock containing at least olivine 90%.

Olivine is the first mineral to be crystallized when a magma cools. This is why it is often present in basalts. It can crystallize at a temperature of approximately 1.000°. It is the first mineral of the reactional continuation: Olivine (Mg) --> Olivine (Fe, Mg) --> Pyroxene (Mg) --> Pyroxene (Fe, Mg) --> Amphiboles --> Biotite (of the HT high temperatures towards BT low temperatures).

The forsterite is never associated with quartz; in fact, the simultaneous presence of forsterite quartz would comprise the spontaneous formation of Pyroxène.

Olivines richer in iron can coexist with quartz in some Granite S and quartziferous Syénite S, where they are associated with other iron-bearing silicates, like pyroxene Hédenbergite CaFeSi₂O₆, pyroxene ægyrine NaFeSi₂O₆ and the Amphibole Arfvedsonite Na₃ (Fe²⁺₄Fe³⁺) Si₈O₂₂ (OH) ₂.

Metamorphic olivines are less common, but are nevertheless important minerals in certain metamorphosed impure marbles and the ultramafic rocks.

Being formed at high temperature and in absence of water, olivines are very sensitive to the atmospheric agents, hydrothermal deterioration, the Métamorphisme of low degree implying the hydration, oxidation, silicification or the carbonation. They deteriorate in serpentine, Chlorite, Amphibole, iron oxides, Talc. Olivines answer perfectly the law of Goldich : “the minerals are all the more vulnerable as their conditions of genesis different more those which reign on the surface”.

Polymorphism

Mg₂SiO₄ is presented with structure olivine in the earth's crust and until the upper part of the zone of transition from the coat. In the middle of the zone of transition, in the 520 depth km neighborhoods, olivine is transformed into Wadsleyite or β-Mg₂SiO₄ with structure modified spinel, containing Si₂O₇ groups. With more important depths, at the base of the zone of transition from the coat, it is the Ringwoodite or γ-Mg₂SiO₄, with structure Spinelle, which becomes stable.

In the series Fo-F one does not observe polymorphism and even less common members (nickeliferous e.g.) have structure olivine.

Layer

• In France, close to Langeac and to the Puy-en-Velay in Haute-Loire and with Allanche in the Cantal.
• On the island of the Meeting to the Piton of the Furnace, one finds in castings of Basalte of the olivine macro-crystals.
• One also finds some on the island of Lanzarote.

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