 # Optical Data

Search Webmineral : ## Optical Data

### Optical Nomenclature used in this database.

 A B C Optical Name Explanation Crystallography a (NX) b g Biaxial (?) Biaxial Optics, Sign Unknown Triclinic Monoclinic Orthorhombic a b g Biaxial (+) Biaxial Positive Optics a b g Biaxial (-) Biaxial Negative Optics a b g Biaxial (+/-) Biaxial Optics, Sign can be either positive or negative n (N) - - Isotropic Isotropic Optics Isometric Amorphous a b - Uniaxial (?) Uniaxial Optics, Sign Unknown Tetragonal Hexagonal Trigonal w e - Uniaxial (+) Uniaxial Positive Optics e w - Uniaxial (-) Uniaxial Negative Optics a b - Uniaxial (+/-) Uniaxial Optics, Sign can be either positive or negative

The value of the index of refraction can be calculated using chemical and physical property data if these values are missing.

The relationship between chemical composition, density, and refractive index was proposed as a means of examining gasses and solutions (Gladstone, Dale(1863), Phil. Trans, 153, 317).  This relationship is as follows:

KP = (n - 1) / d = constant

Where

n = mean index of refraction

D = density

The practice of using the Gladstone-Dale relationship to minerals only gives an approximation because of the effects that different crystal systems have on the anisotropy of the crystal lattice and the resultant values of n (index of refraction). The value of KP may be estimated from the value of Kc, the value of the Gladstone-Dale constant derived from the chemical composition.

KC = SUM OF (k1p1/100 + k2p2/100... + knpn/100). The Gladstone-Dale constant estimated from chemical analysis

Where
kc = Gladstone-Dale constant of chemical phase "n"
kp = Percentage of the chemical phase "n"

### Birefringence

Birefringence is an optical property possessed by a material which has more than one index of refraction. This anisotropy in the index of refraction is dependant on the crystallographic projection and can be calculated as follows:

• Uniaxial Minerals: Birefringence = abs(NO - NE).
• Biaxial Minerals: Birefringence = NZ - NX.

Birefringence can also be estimated from a color interference chart for colors observed under crossed polarizers on a petrographic microscope. The interference color chart is from Bloss, 1962, fig. 8-17. Click on the image for a larger picture. ### Two V Calculated

TwoV(Calc) is the Biaxial 2 V angle calculated from the following relationship:

```Function TwoV(A, B, C)
' From Bloss, 1962, p. 156
'
'         (gamma+beta)(gamma-beta)     alpha
'    (sqr(--------------------------)) -----  = cos(Vz)
'         (gamma+alpha)(gamma-alpha)   beta
```

where gamma=Nz, beta=Ny, and alpha=Nx

### Dispersion

Dispersion is defined as the separation of a ray or beam of white light into its component colors. The common example is the separation due to a glass prism into the colors of the rainbow. Dispersion can be measured by calculating the index of refraction at different wavelengths. High dispersion materials separate the colors more effectively then low dispersion materials. High dispersion provides the "fire" in gemstones. Low dispersion materials are desirable in optical lenses.

In the special case of biaxial minerals, dispersion is noted by the color fringing in acute bisectrix figures using a petrographic microscope. This fringing is due to the variation of the refractive indices of α, β, and γ (Nx, Ny, Nz) with respect to wavelength. The net result is the variation of 2V angle with wavelength. This is represented by the relative position of 2Vr for red light and 2Vv for violet light on the opposite ends of the visible spectrum. This is shortened to r and v

• When 2Vr is less than 2Vv then it is noted as r < v.

• When 2Vr is greater than 2Vv then it is noted as r > v.

• Modifiers to r versus v include:
none for no noticeable dispersion
weak for weak dispersion
moderate for moderate dispersion
strong for strong dispersion
extreme for maximum dispersion ### Other References to Optical Data

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Example Subject Searches

Example: biaxial1.7* finds all biaxial minerals with the lowest index of refraction 1.7 to 1.79.
Example: biaxial1.7* bire=0.015* pleochroism "pale blue" finds all biaxial minerals with the lowest index of refraction from 1.7 to 1.799, a birefringence of 0.0150 to 10159 and a pale blue pleochroism.
Example: uniaxial1.6* finds all uniaxial minerals with the lowest index of refraction 1.6 to 1.69.
Example: isotropic1.5* finds all isotropic minerals with the lowest index of refraction 1.5 to 1.59.

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