Physically, crystals are usually short columns along the C axis, which are aggregates of {1 10}, {100}, {00 1} and {02 1}, and are often granular aggregates. The color is olive green, light magnesium-rich, often with yellow tone and deep iron-rich, and it is yellow, brown or red after varying degrees of alteration. Glass luster, fracture grease luster. The cleavage along {0 10} is moderate or incomplete. The hardness is 6 ~ 7 and the relative density is 3.3 ~ 4.4, which increases with the increase of iron content.
Fig. 13- 1 olivine optical orientation diagram
Optical feature sheets are colorless, with high protrusions, parallel extinction, and sometimes ring-shaped structure, and twins are rare. There is a strong birefringence between orthogonal polarizations, and the second-order interference color reaches the bottom of the third order; Ductility can be positive or negative; Optical symbols are related to iron content. Those with less iron have positive optical properties and large optical axis angles. With the increase of iron content, the optical axis angle can reach 90. If the iron content continues to increase, it will be transformed into negative optical properties, and its optical axis angle is quite large (always close to 90), so in the interferogram perpendicular to the optical axis angle, the black arm is similar to a straight arm. The optical direction is shown in figure 13- 1.
The identification features are granular and self-form is rare; Characteristic olive green, glass luster, obvious ghosting phenomenon can be seen; Normal corrosion becomes serpentine, tremolite, talc, soapstone, asbestos, carbonate minerals, etc. Anisotropic body with high elevation and obvious surface roughness; {0 10} and {100} cleavage is incomplete, and cracks develop. Twins are rare; Grade Ⅱ end-grade Ⅲ interference color; Parallel extinction; Ductility can be positive or negative; Biaxial crystal, (+) 2v = 85 ~ 90, (-) 2v = 50. Positive light or negative light, multicolor is weak yellow-green; Refractive index1.654 ~1.690 (0.020), birefringence 0.035 ~ 0.038, density 3.34(+0. 14,-0.07) g/cm3; The hardness is 6-7; Sometimes we can see a group of imperfect cleavage.
Occurrence and other olivines are the main minerals that constitute the upper mantle, and they are also the main mineral components of meteorites and moonstone. As the main rock-forming mineral, it mainly occurs in basic rocks and ultrabasic rocks containing unsaturated silica, and generally does not occur at the same time. Forsterite is also produced in magnesium skarn. Olivine is unstable in nature and becomes serpentine under the action of heated liquid, so it rarely appears in metamorphic rocks and sedimentary rocks.
In the past, the origin of olivine was near Garde Island in Zabal in the Red Sea, and the name "olivine" was also related to the name of this place. However, at present, olivine in the mining market mainly comes from Myanmar, the United States and China. Hebei, China is an important producing area of olivine in the world, and there are also commercial producing areas in Northeast Heilongjiang Province.
Transparent, bright and flawless olivine crystals can be used as gems. The ancient Egyptians used olivine as jewelry in BC, and the ancient church in Cologne, Germany was also inlaid with high-quality olivine gems. Olivine is also one of the few characteristic gems in China.
As a gem, the price of olivine is not only related to the depth of green, but sometimes its particle size also has a great influence. Because there are many cracks in the raw materials of olivine, there are few raw materials that can grind big gems, and the output of gems is low, so the quality of workmanship is also a factor that affects the price. Olivine has moderate hardness and cannot be put together in large quantities, otherwise the edges and corners will be rounded, which will affect the appearance and price.
There are many gemstones similar to olivine, such as green tourmaline, green beryl and green emerald. But there is often only one kind of glass pretending to be olivine on the market. The difference between olivine and glass can be seen from the obvious "double shadow" of olivine, while glass does not; Sometimes crystal inclusions can be seen in olivine, but there are only bubbles in glass. The key to evaluate olivine is deep and pure color, no crack and large particles.
Experimental course of crystal optics