Olivine is an ancient gem variety in the west. In 1500 BC, it was mined on an island in the Red Sea and used as a gem. Olivine is also called Zabargad, according to the name of the island. In Arabic, the word is olivine, and the world's largest olivine (365,438+00 carats) comes from this island. For thousands of years, the soft and beautiful color of olivine has been deeply loved by people. It has always been the birthstone of August, symbolizing peace, friendship, happiness and harmony in the family.
First, the basic characteristics of olivine
Mineral Name: Olivine
Chemical composition: chemical formula (Mg, Fe)2[SiO4]. Gem-grade olivine contains more magnesium than iron, and Mg2 [SiO _ 4] molecule is generally 92% ~ 88%. It may contain trace amounts of manganese, nickel, cobalt, chromium and titanium.
Crystal system and crystallization habit: orthogonal crystal system. Columnar, flat columnar, granular columnar or round, irregular granular (figure 16-8- 1).
Ideal crystal form of olivine
Optical characteristics: Gem-grade products are light green yellow, yellow green (olive green), green brown and brown, and the color and multicolor change with iron content. Light color is not multicolored, green is green-light green yellow-green, and brown is brown-light brown-dark brown. This kind of glass is shiny and transparent. Starlight effect or cat's eye effect is rare.
The biaxial crystal is positive, and the excess iron becomes negative. The refractive index is generally 1.654 ~ 1.690. The birefringence is 0.035 ~ 0.038, and the faceted stone can see pavilion prism double shadow and inclusion double shadow from the table top. The dispersion (0.020) is moderate.
Olivine has no fluorescence reaction under ultraviolet light. It is green under the Charles filter. The characteristic absorption spectra are three narrow absorption bands in the blue region, which are located at 433nm, 474nm and 497nm respectively.
Mechanical properties: {100} and {0 10} cleavage is incomplete, conchoidal fracture. The hardness is 6.5 ~ 7 and the toughness is medium to good. Density: Gem grade. High heat sensitivity, easy to break when heated unevenly or too fast. Rapid reaction with HCl, HF or concentrated hot H2SO4; Can't use any acid, alkali cleaning.
Microscopic characteristics: Gem-grade olivine is basically magmatic, and its typical inclusions are lava glass beads of different sizes and round, oblate and lotus-shaped residual (magma) gas-liquid films formed around chromite microcrystals or negative crystals, characterized by "water lily pad" or sea lily-like inclusions (there are star-shaped beam-like burst cracks around chromite microcrystals or negative crystals). The separated mineral guest crystals include chromite, brown to reddish-brown chrome spinel, reddish-brown biotite, chrome diopside and graphite.
Second, the type of olivine.
There are different types of olivine. Generally, it is divided into two categories according to color, mainly yellow-green to green-yellow, and mainly brown. There are also some that don't belong to the same category, because brown olivine is relatively rare, and there are few gem-grade markets.
The following species division, although the name is applied to the names of mineralogical olivine species and subspecies, is not the meaning of these two terms in mineralogy at all, but the names of commercial varieties that are not common in the world.
(1) Olivine: refers to medium to dark green yellow gem-grade olivine.
(2) Expensive olivine: refers to yellow-green and light green-yellow gem-grade olivine.
Thirdly, the identification of olivine.
Similar in appearance to olivine are green tourmaline, diopside, zircon and glass. It can be distinguished from regional glass, yttrium aluminum garnet, synthetic spinel, plastic and fluorite by polarization or double shadow phenomenon. It can be distinguished from diopside, emerald, tourmaline, chrysoberyl and apatite by refractive index and birefringence.
It is pyroxene that overlaps with the optical constant of olivine, so it is effective to accurately measure birefringence and density. When the inlay density can't be measured, it is also helpful to compare the differences of inclusion, polychromatic and absorption spectrum.
Rare gemstones embedded with or similar to olivine optical data, such as boehmite and zoisite, need to be carefully selected to distinguish the test items. The density of boehmite is higher than that of olivine, and it is polychromatic, mainly brown, with four absorption bands of 493nm, 475nm, 463nm and 452nm, while olivine has no narrow band of 463nm. Zoisite birefringence is low, ranging from 0.009-0.0 10, and the absorption spectrum is only a diffuse weak band, and generally only the last two can be seen at 595nm, 528nm and 455nm.
Four. Evaluation of olivine
Olivine wins with its soft color, and the most pleasing color is medium to dark green yellow (olive green), which is as elegant as emerald but not as rich as emerald. Because of the good transparency, it is easy to see the dirty lock with the naked eye, so it is not suitable to use inclusions as gems or low-grade products.
Pure green is rare and of high value. The particle size (above 3 carats) is less and the value is higher.
Brown or brown-yellow-green, although rare, but of low value, is mostly collected.
A brief introduction to the occurrence and genesis of olivine deposits.
Olivine has two genetic types: magmatic type and metamorphic type. Gem-grade olivine is basically magmatic, and its main occurrences are:
(1) peridotite xenoliths in basalts: olivine in mantle rocks captured by basalt and megacrysts of olivine scattered in basalt. Some people think that the latter is brought to the near surface after crystallization in deep-cold magma, while others think that the magma reaches the near surface and crystallizes rapidly with the rapid volatilization of fluid, which is a large and clean crystal.
(2) Olivine in peridotite hydrothermal veins: It occurs in peridotite strong serpentine veins.
At present, olivine in SanCarlos, Arizona, USA is the largest source of gem-grade olivine; Beautiful olivine with trace chromium is produced on the coast of Hawaii. Zebargad, an ancient and famous Egyptian producing area, produces little, but olivine has a large particle size, and there are still several centimeters of gems.
Other famous producing areas are Australia, Brazil, Kenya, Tanzania, Norway, Sri Lanka and Myanmar; Myanmar used to be a famous olivine producer, and it also produced some large particles of olivine. The former Soviet Union and Czech Republic also produced some olivine.
Two famous olivine producing areas in China are inclusions in basalt. The output of Zhangjiakou in Hebei is greater than Jiaohe in Jilin. The particle size of olivine in Damaping, Zhangjiakou is generally 5 ~ 7 mm, and the maximum is 34mm×20mm× 15mm, showing olive green and yellow-green. The particle size of olivine in Baishishan of Jiaohe River is mostly 5 ~ 8 mm, and that of olivine is generally10 ~ 20 mm. The study of absorption curve shows that the main color is yellow, and the appearance is yellow-green or yellow-green.