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:

(n - 1) / d = constant

Where

n = mean index of refraction

D = density

constant = Gladstone-Dale constant

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).

Compatibility Index

Mandarino, in his review of the Gladstone-Dale relationship in minerals (Can,
Min, (1981), Vol 19, pp 441-450) proposed the concept of the Compatibility Index
in comparing the physical and optical properties of minerals. This compatibility
index is a required calculation for approval as a new mineral species (see IMA
guidelines).

The compatibility index (CI) is defined as follows:

CI meas = (1 - KPDmeas / KC )
CI calc = (1 - KPDcalc / KC )

Where

KP = Gladstone-Dale Constant derived from
physical properties

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"

The internal consistency of the Compatibility Index is defined by the
following Table of values which describe the "Goodness" of the mineral
data.

Compatibility Index

Category

± 0.000 to ± 0.019

Superior

± 0.020 to ± 0.039

Excellent

± 0.040 to ± 0.059

Good

± 0.060 to ± 0.079

Fair

> ± 0.079

Poor

Summary

The calculation of the Compatibility Index is based on the values of the chemical composition,
index of refraction, and calculated or observed density for each mineral species
and is included in the optical section of the mineral species description.

The following graph illustrates the application of Mandarino's Compatibility
Index (CI) to the minerals in this database:

Of
all the minerals (2,679) where the CI is applicable (excluding elements,
sulfides,
halides, and bad oxide analysis), there are 2,180 minerals with a CI of 0.079 or
better.

Conversely, 18.6% of of the 2,679 minerals have a CI
rating
of "Poor".