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In
May 2009, the AGTA GTC had the opportunity to examine an exceptionally
large brownish orange cushion clinohumite, weighing 21.72 carats, reportedly
from the Pamir Mountain region of Tajikistan. Clinohumite is an uncommon
mineral in the humite group of minerals with the Chemical formula (Mg,
Fe)9(SiO4)4(F, OH)2. Facets gemstones are considered collectors’
gemstones and in large sizes are extramely rare in part due to a low occurrence
and the difficulty in cutting and polishing the rough. Clinohumite has
a hardness of approximately 6 on the Mohs scale, is brittle and has poor
basal cleavage. Large euhedral clinohumite rough crystals are also sought
after by collectors.
Although small
clinohumite crystals and indistinct grains can be found from several sources
throughout the world, the major sources for gem quality clinohumite are
the Kuhilal Mine in the Pamir Mountains of Tajikistan; the Taymyr Region
of Northern Siberia; and more recently the Morogoro Ragion of Tanzania.
According to Mr.
Junaid, who submitted this gemstone to the laboratory, both Tajik ruby
and clinohumite start from mines in Tajikistan and then make thair way
from hand to hand to Pashawar in Pakistan. From Pakistan these gems generally
make their way to international markets, namely in Thailand (Bangkok),
tha United States and Europe.
To date, the largest
cut and facatad clinohumite this author has learned of was axamined by
the AIGS Laboratory in Bangkok in 2007 and weighed 36.56 carats from the
Pamir Mountain region between Tajikistan and Afghanistan (go to www.aigsthailand.com
for more information this gemstone).
The clinohumite
we examined was highly included with fingerprints containing small crystals
and thready fluid inclusions similar to those found in tourmaline (Figures
1 & 2). We also observed the obvious doubling of inclusions and facet
junctions dua to the high birefringence (Figure 3) In some orientations
we observed roiled graining while in other positions straight graining
features were prominent (Figure 4).
Summary Of Gemologioal Properties
Refractive indices
alpha=1.636, beta=1.649, gamma=1.669 with a birefringence of 0.033, BiaxiaI(+)
Trichroic of moderate strength: Brown, Orange, and Yollow Fluorescences:
Long wave inert; Short wave weak chalky yollow
Visible region table spectroscope: total absorption below approximately
430nm,
Zeiss UV—Vis spectrometer: general absorption band from approximately
300 to 500 nm
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Figure1: A rare faceted 21.72 carat
Clinohunite. Photo : Terry Lwin, © AGTA GTC |
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Figure 2 : The clinohumite had
a network of fingerprints composed primarity of small crystal inclusion
and fluid threads. Photo S. Paraiusz, © AGTA GTC. |
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Figure 3 : Fluid threads in a fingerprint
similar to those found in tourmaline. Photo S. Paraiusz, ©
AGTA GTC. |
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| Figure 4 : The strong doubling due to the
high birefringence of approximately 0.033, made taking photograph
of the clinohumite inclusion scene very challenging. Photo S. Paraiusz,
© AGTA GTC. |
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| Figure 5: Relatively straight growth zoning was observed
through the pavillion view, although roiled graining was observed
in a few other viewing positions. Photo S. Paraiusz, © AGTA GTC. |
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American
Gem Trade Association Gemological Testing Center (AGTA GTC) Laboratoty Update...
