16.27: Clinohumite

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gemology
Gemology Project

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Clinohumite
Chemical composition	Magnesium silicate
Hardness	6
Optic nature	Biaxial +
Refractive index	1.628 - 1.674
Birefringence	0.024 - 0.041
Specific gravity	~ 3.21
Lustre	Vitreous
Pleochroism	Strongly trichroic
Fluorescence	SWUV: strong orange yellow LWUV: generally inert

Figure \(\PageIndex{1}\): A 9.45 ct brownish orange clinohumite tested at the Gem Testing Laboratory, Jaipur.

Clinohumite is a member of the humite group. As a gemstone it is rare, but some cut stones are reported. The given below details are (largely) abstracted from Journal of Gemmology, 2007, 30, 5/6, 303-306 "A remarkably large clinohumite".

Crystallography

Monoclinic

Diagnostics

Color

Bright yellow-orange color resembling some hessonite and spessartine garnets.
Cause of color: Mn

Diaphaneity

Transparent

Refractometer

Clinohumite has a biaxial optic character with a positive optic sign.
The refractive index range of clinohumite: n_α = 1.628 - 1.638, n_β = 1.641 - 1.654, n_γ = 1.662 - 1.674.
The maximum birefringence ranges from 0.024 to 0.041.

For a specific stone tested (9.45 ct. brownish orange clinohumite tested at the Gem Testing Laboratory, Jaipur): R.I.: 1.646-1.670, birefringence: 0.024.

Pleochroism

Strong trichroism: yellow, orange and brownish yellow.

Specific gravity

S.G.: ~ 3.21

Spectrum

Figure \(\PageIndex{2}\): Diffraction spectrum

Brownish-orange clinohumite.
General cut-off till 430nm (from 400nm).

Luminescence

SWUV: strong orange-yellow;
LWUV: generally inert

Magnification

Inclusions: Strong color and growth zoning, flat parallel reflective planes, fluid-like inclusions similar to trichites in tourmaline. Such liquid inclusions along with doubling appear very much like tourmaline; tube like inclusions; sometimes swirly zones.

Occurrence

Geographical occurrence: Gem quality Clinohumite is known to occur in two important localities - The Pamir mountains (at Kukh-i-lal, Sumdzin and Changin); in Tajikistan in association with spinel and in the Taymyr region (Basin of Kotui river) in Siberia; Also in Mahenge, Tanzania.

References

G.Choudhary and Chaman Golecha, A remarkably large Clinohumite, Journal of Gemmology, 2007, 30, 5/6, pp 303-306
Henn, U., Hyrsl, J., and Milisenda, C.C., Gem Quality clinohumite from Tajikistan and the Taymyr region, Northern Siberia. Journal of Gemmology, 27(6), 335-9
Hyrsl, J., 2001. Gem News International. Spinel with clinohumite from Mahenge, Tanzania, Gems and Gemology, 37(2), 144-5
Laurs, B.M., and Quinn, E.P., 2004. Gem News International. Clinohumite from Mahenge, Tanzania. Gems and Gemology, 40(4), 337-8

Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster (6th ed.)
Colour Encyclopedia - J.E. Arem ISBN 0412989115
Introduction to Optical Mineralogy 3rd edition (2003), Prof. W.D. Nesse

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