Lets Talk Gemstones
By Edna B. Anthony, Gemologist
(Contact the author for
permission to reproduce this article in any form.)
P.O.# 62653; COLORADO SPRINGS,
CO. 80962
HUMITE GROUP
[NESOSILICATES]
CHONDRODITE, HUMITE, CLINOHUMITE and NORBERGITE
One year during a visit to the famed Tucson show, the author discovered a
startlingly beautiful golden-yellow faceted gemstone identified as clinohumite,
purported to be from the Lake Baikal region in Siberia. The dealer could
provide little information about the gem’s physical and optical properties.
Thus began a search to determine how practical its use for jewelry might
be.
The Mineralogical Record, Inc. publication Glossary of Mineral Species lists
alleghanyite, chondrodite, clinohumite, humite, jerrygibbsite, leucophoenicite,
manganhumite, norbergite, rubellite and sonolite as members of the humite
group. Walter Schumann mentions none of these in his Gemstones of the World.
Of the ten species, only chondrodite, clinohumite and norbergite are listed
in Michael O’Donoghue’s American Nature Guides Rocks and Minerals. Dr. Joel
Arem adds humite to the list in his Color Encyclopedia of Gemstones. The
discussion of the humite group in Cornelius S. Hurlbut, Jr. and Cornelis
Klein’s Manual of Mineralogy after J. D. Dana tells us the members of the
group are so similar in chemistry, structure, and physical characteristics
that optical tests must be used to distinguish one from another. A structure
that alternates layers composed of Mg(OH,F)2 with others, which possess the
atomic arrangement of olivine, create a close relationship to the structure
of olivine. Hydroxyl end members are not known to occur, although the replacement
of iron by OH can be extensive. Each specie is biaxial positive and exhibits
a vitreous luster. Poor cleavage and infusibility are common to all. Exposure
to long-wave ultra-violet light reveals a dull orange luminescence. While
most specimens remain inert under short-wave light, the exceptions exhibit
a bright yellow fluorescence. TiO2 in humite and clinohumite greatly influences
their optical properties.
All of the species develop in contact regions of metamorphic metasomatised
limestone and dolomites and in skarn derived from such deposits, into which
large amounts of magnesium, iron, aluminum and silicon have been introduced.
They are seldom found in alkaline formations of igneous origin. Dr. Arem
mentions a source in a carbonatite formation in Loolekop, East Transvaal
in Africa, but he does not specify if the formation is of igneous magmatic
or sedimentary origin. The American Geological Institute’s Glossary of Geology
states that although “carbonatite” has been used synonymously with “limestone,”
the more common use of the term designates an igneous rock. Wiberforce, Ontario,
Canada is a source of crystals of the humite group, and deposits in Pargas,
Finland and Kafveltorp, Orebro, Sweden produce yellow material. Noted sources
of the specific species are listed in the narratives below. The characteristic
small crystals of the group often harbor numerous inclusions and fractures,
so faceted gems larger than three carats are extremely rare.
CHONDRODITE
This most common member of the humite group [Mg5(SiO4)2(F,OH)2] usually occurs
in granular and massive forms. Its tiny transparent to translucent yellow,
red and brown monoclinic crystals frequently show lamellar twinning. The
Manual of Mineralogy tells us its association with graphite, phlogopite,
pyrrhotite and spinel in crystalline limestone formations is highly characteristic.
It occurs with fosterite, monticellite and wollastonite in skarn deposits.
The Tilly Foster magnetite deposit near Brewster, New York yields most of
the available gem crystals, although it is also found in Kafveltorp, Sweden,
the Pargas area in Finland and Monte Somma, Italy. The density can vary from
3.16 – 3.26. It possesses a hardness of 6.5. Refractive readings range from
1.592 to 1.646 with a birefringence variation from 0.028 to 0.034. Dr. Arem
lists pleochroism colors of very pale yellow/brownish yellow – colorless/yellowish
green – colorless/pale green. The combination of toughness and good hardness
with rich colors would make chondrodite a lovely gemstone, but the lack of
size makes the few faceted gems prized additions to a collector’s cabinet.
NORBERGITE
The granular form and the frequently highly modified yellowish to orange
and brown transparent to translucent crystals of norbergite [Mg3(SiO4)(F,OH)2]
develop in the orthorhombic crystal system in contact regions in dolomite
and limestone formations. The name reflects the source located in the Ostanmosoa
iron mine in Norberg, Sweden. The Franklin area of New Jersey also produces
this specie of humite. Scratch tests reveal a hardness of 6.5 on the Mohs
scale. With a range of 3.15 to 3.18, it has the lowest and least variable
density of the four gem humites. Its refractive indices are also the lowest,
with readings of 1.563 to 1.593 and a birefringence of 0.026 to 0.027. The
dichroscope reveals the three pleochroism colors of pale yellow, very pale
yellow, and colorless. Like chondrodite and humite, the crystals are so small
that the very few gemstones cut from this material are the province of collectors.
HUMITE
Orthorhombic humite forms a series with monoclinic clinohumite. Its chemical
formula can be written as Mg7(SiO4)3(F,OH)2. Gemmy, small yellow and intense
orange translucent to transparent crystals are found in association with
clinohumite and chondrodite at the Tilly Foster Mine. Humite exhibits a hardness
of 6 on the Mohs scale, a density of 3.20 to 3.32, and refractive index readings
from 1.607 to 1.675. The birefringence varies from 0.029 to 0.031. Trichroism
colors of yellow - colorless/pale yellow - colorless/pale yellow are seen
with the dichroscope. Suitable rough for cutting is extremely small and almost
unobtainable, so faceted humite is almost unknown. These tiny gems would
certainly be collectors’ items.
CLINOHUMITE
Monoclinic clinohumite [Mg9(SiO4)4(F,OH)4] is an end member of the humite
to clinohumite series The yellow, brown, orange, and white transparent to
translucent crystals can be found in contact zones in dolomite formations,
talc schists, and in serpentines. The Tilly Foster iron mine, the Pargas
area in Finland, and the Llanos de Junar, Malaga, Spain are sources of this
specie, but the largest and finest intense yellow and orange crystals are
recovered in the Pamir Mountains and the Lake Baikal region of Siberia. At
refractive indices of 1.631 to 1.668 and a birefringence of 0.037, the optics
of the material from Siberia vary from the normal refractive readings of
1.629 to 1.674, with a birefringence range of 0.028 to 0.041. When viewed
through the dichroscope, clinohumite’s pleochroism colors of golden yellow/deep
reddish yellow - pale yellow/orange yellow and pale yellow/orange yellow
are revealed. A variation in density can range from 3.17 to 3.35, with a
hardness of 6.
Gems cut from clinohumite are not common, but they can be larger and more
abundant than gems of its sister species. They possess excellent toughness,
a fair hardness, and bright intense colors. For those who appreciate the
unique, one set in an appropriate ring mount and treated with care or a pair
featured in earrings would be an asset to one’s collection of jewelry.