Peridot (gem)

Species: PERIDOT

Color: Green, yellow-green, green-brown

Transparent: Transparent

Crystal system: Rhombic

Chemical formula: (Mg,Fe)2(SiO4)

Chemical composition: Iron magnesium silicate

Refraction: Positive or negative biaxial birefringent

Refractive index: 1.654-1.672-1.690 (+0.017/-0.004)

Birefringence: 0.036 (±0.002)

Dispersion: 0.020

Pleochroism: Very weak, yellow-green and green

Density: 3.34 (+0.15/-0.07)

Hardness: 6 ½ - 7

Habit: Prismatic

Genesis of deposits: Magmatic

Main deposits: Australia (Queensland), Brazil (Minas Gerais), China (Hebei), Egypt (Zebirged), Kenya, Myanmar (Mogok), Mexico, Norway (Sondmore), Sri Lanka (Ratnapura), United States (Arizona, Hawaii) South Africa.

Technical specifications

The origin of the term peridot is uncertain; it is supposed to derive from the Greek language with the meaning of "abundance", in reference to the numerous faces present in its crystals. Peridot is an isomorphic mixture of forsterite (Mg2Si04) and fayalite (Fe2Si04) which, in different percentages, generate a series of crystals known in mineralogy as olivine. These minerals take on different names depending on the Mg/Fe ratio; starting from the magnesite terms to conclude with those rich in iron, six terms can be distinguished: forsterite, chrysolite, hyalosiderite, hortonolite, ferrohortonolite, fayalite. Peridot represents the transparent variety used as a gem and usually belongs to the term chrysolite, being richer in magnesium than in iron. In the past, the word chrysolite referred to peridot with a richer yellow color. Olivines belong to the nesosilicates and as such are composed of isolated Si04 tetrahedra, linked together by iron and magnesium atoms in octahedral coordination with oxygens. Peridot is found in igneous rocks formed during the early stages of magmatism and is the main constituent of some rock formations such as peridotites. The presence of this mineral has also been found in meteorites and lunar rocks. The entire olivine group is easily altered into minerals such as serpentine and chlorite. The habitus of the crystals is prismatic although these are more frequently found in rounded nodules. A cleavage according to the pinacoid is rarely found, the fracture is conchoidal and the luster is vitreous. Peridot crystals fracture with a certain ease both under pressure and under rapid heating; for these reasons it is advisable to clean it only with warm soapy water and never with ultrasound or steam. Peridot is susceptible to attack from hydrochloric and sulfuric acid while it is stable to prolonged exposure to sunlight. The coloration derives from the presence of iron and for this reason the mineral could be defined as idiochromatic, but it seems that the shade is influenced by impurities of chromium, often present, in octahedral coordination, together with nickel.

Usually this gem is not subjected to treatments even if, with heating, it is possible to lighten some samples particularly rich in iron. Since the last century, attempts were made to obtain the synthesis of the mineral but, given the low commercial value, it was not considered appropriate to develop this project. Due to its color and the values ​​of the refractive indexes, peridot can be confused with other biaxial birefringent gems, such as: hiddenite, cornerupine, diopside and sinhalite. Peridot is distinguished by having the lowest refractive index 𝜶, for the high birefringence, for the absorption spectrum and, when present, for the characteristic inclusions. It is not always possible to trace the optical sign of this gem since its B index is close to half of the interval 𝜶-𝜸. It should be emphasized that for small variations of the intermediate index, optically positive or negative samples can be obtained.

Internal features

Being a mineral not very rare, peridot can give rise to gems with few or no inclusions. The most typical of these are those defined, in English Lily pads, surrounded by discoidal cleavages. These circular halos are formed by tensions generated by the different thermal expansion of the included crystal and can be subsequently filled by healing liquid inclusions that appear as circular fins with a fine and orderly mesh. When liquid inclusions heal other types of fractures, they can generate fins of different shapes. Peridots often present nebulous trails, similar to smoke, of a silvery white color if illuminated with a dark field, due to tensions caused by zones of incomplete solid solutions. Among the solid inclusions the most frequent are: spinel, in octahedral crystals, reddish brown; chromite, black; biotite, in brown sheets and diopside in green crystals.

The volcanic origin of some peridots is evidenced by the presence of solidified amorphous zones that are defined as glass drops. In some peridots of meteoric origin, perpendicularly crossed acicular inclusions have been observed, probably produced by exsolution phenomena due to variations in temperature and pressure.