Peridot
Forsterite
A variety of Olivine Group
What is Peridot?
When peridot is polished up and cut into attractive shapes, it is more readily known as peridot. Traces of it have also been found in meteorites and collected from space dust, making this mineral extraterrestrial as well! It is currently being researched for use in the medical field for transplants.
Etymology & Origins
It was named by Armand Lévy in 1824 after the English naturalist and mineral collector Adolarius Jacob Forster.
Uses & Applications
Peridot is a mineral that is highly valued in its gemstone form, which is called peridot. But there are no current industrial or commercial uses.
Healing Properties
You won't reach your wit's end with peridot, as it's said to strengthen patience. It's believed that peridot will help you remain calm due to its pacifying effect on your energy. It is supposedly a stone of productivity, giving you the motivation to complete all that's needed of you, maybe even more!
Optical Properties
- Refractive Index
- 1.635-1.671
- Birefringence
- 0.034-0.035
- Optical Character
- Biaxial positive
Discover Values
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Key Characteristics
Rock Types of Peridot
Two polymorphs of forsterite are known: wadsleyite (also orthorhombic) and ringwoodite (isometric). Both are mainly known from meteorites. Peridot is the gemstone variety of forsterite olivine.
Formation of Peridot
Each oxygen atom is bonded to the silicon by a single covalent bond. The four oxygen atoms have a partial negative charge because of the covalent bond with silicon. Therefore, oxygen atoms need to stay far from each other in order to reduce the repulsive force between them. The best geometry to reduce the repulsion is a tetrahedral shape. The cations occupy two different octahedral sites which are M1 and M2 and form ionic bonds with the silicate anions. M1 and M2 are slightly different. M2 site is larger and more regular than M1 as shown in Fig. 1. The packing in forsterite structure is dense. The space group of this structure is Pbnm and the point group is 2/m 2/m 2/m which is an orthorhombic crystal structure. One of the important factors that can increase the portion of forsterite in the olivine solid solution is the ratio of iron(II) ions to iron(III) ions in the magma. As the iron(II) ions oxidize and become iron(III) ions, iron(III) ions cannot form olivine because of their 3+ charge. The occurrence of forsterite due to the oxidation of iron was observed in the Stromboli volcano in Italy. As the volcano fractured, gases and volatiles escaped from the magma chamber. The crystallization temperature of the magma increased as the gases escaped. Because iron(II) ions were oxidized in the Stromboli magma, little iron(II) was available to form Fe-rich olivine (fayalite). Hence, the crystallizing olivine was Mg-rich, and igneous rocks rich in forsterite were formed. At high pressure, forsterite undergoes a phase transition into wadsleyite; under the conditions prevailing in the Earth's upper mantle, this transformation would occur at pressures of ca. 14–15 GPa. In high-pressure experiments, the transformation may be delayed so that forsterite can remain metastable at pressures up to almost 50 GPa.
Composition of Peridot
Pure forsterite is composed of magnesium, oxygen and silicon.
Quick Facts
Physical Properties
- Color
- Green, pale yellow, white
- Hardness (Mohs)
- 7
- Density
- 3.271 g/cm³
- Streak
- White
- Luster
- Vitreous
- Crystal System
- Orthorhombic
Chemical Properties
- Chemical Formula
- Mg2SiO4
- Elements
- Mg, O, Si
Also Known As
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