What is the refractive index in mineralogy ?

Refractive index : definition

The refractive index of a material (mineral or other) is defined by n = V0 / Vmin , where V0 is equal to the speed of propagation of light in vacuum and Vmin the speed of light in the mineral. It is a dimensionless value, it therefore reflects the slowing undergone by the light during its penetration into the body. Its value is usually between 1.3 and 2.6 for minerals. It is measured with a refractometer ideally on a flat and polished surface (the table of a faceted gemstone, a crystalline face).

This parameter largely determines the luster of minerals : a vitreous or related luster indicates an index between 1.3 and 1.9, an adamantine luster an index between 1.9 and 2.6, a sub-metallic luster an index between 2.6 and 3 and finally a metallic luster, an index greater than 3.

In the majority of minerals (except all those belonging to the cubic system and therefore isotropic), the refractive index varies according to the direction of propagation : these minerals are said to be birefringent, the incident ray splitting into 2 refracted rays. This property is the basis of crystal optics and the study of minerals by microscopy.

Some examples of refractive index :

Diamond : 2,4
Sphalerite : 2,3
Cerussite : 2
Zircon : 2
Garnet : 1,78-1,88
Corindum : 1,77
Peridot : 1,65-1,69
Tourmaline : 1,62-1,64
Topaz : 1,61-1,62
Beryl : 1,57-1,60
Quartz : 1,54-1,55
Fluorite : 1,43
Opal : 1,40-1,46

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