Lets Talk Gemstones
By Edna B. Anthony, Gemologist
Please contact the author for permission
to reprint this article.
Axinite: A Cyclosilicate Group
On an excursion in the Alps
in 1797, mineralogist, R. J. Hauy discovered some highly vitreous, piezio-electric
wedged-shaped crystals that resembled schorl, the dark variety of tourmaline.
The incorrect designation, "vitreous schorl", was used even after axinite
(Ca,Mn,Fe,Mg)3Al2BSi4O15(OH) was identified as a complicated borate silicate
group of minerals. The characteristic axe shape of its crystals gave it
its name.
If magnesium is the predominant
element of the incorporated ions into its four-member tetrahedral "ring"
silicate structure, then it is magnesioaxinite. In ferroaxinite, the iron
content is greater than manganese. Manganaxinite is the result if manganese
exceeds the iron content. In tinzenite, manganese exceeds the iron content,
and calcium is less than 1.5. Since axinite develops in the triclinic crystal
system with its low degree of symmetry, and its angles are very acute.
Its habit is beautifully illustrated in a picture of rich brown crystals
in matrix on page 67 of the Color Treasury of Crystals by Vincenzo De Michele
of the Natural History Museum of Milan, Italy.
Massive material in lamellar
to granular form is found. Homogeneous growth of the crystals almost always
causes feathers and zoning. An excellent photograph of zoning and large
two-phase inclusions in a yellow-brown Sri Lankan stone can be found on
page 415 of The Photo Atlas of Inclusions in Gemstones by E.J. Gubelin
and J. I. Koivula.
Axinite, a metasomatic (replacement)
mineral, occurs in contact metamorphic areas in basic rocks and veins.
Gem quality material is found in several counties in California, Sri Lanka,
and in the French Alps. Deposits of massive material are located in Luning,
Nevada, Pennsylvania, and New Jersey. Other sources are Cornwall, England,
Finland, Norway, Germany, Japan, Russia, Tasmania, and Mexico. Tinzenite
is found in Switzerland and magnesioaxinite in Tanzania.
Crystals large enough to cut
gems over ten carats in size are extremely rare. Dr. Joel Arem states in
his Color Encyclopedia of Gemstones that "clean stones over 5 carats are
---- worthy of museum display". Despite being a bit brittle, its brilliance
and sumptuous colors make it a unique and exquisite gemstone.
Gemstone Properties
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Silicate {Cyclosilicate}. |
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Manganaxinite,
Tinzenite. |
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By color, species names
& crystallography |
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Colorless, yellow, olive
green, cinnamon brown, violet brown, pale violet to reddish violet, and
blue. |
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Transparent and translucent |
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Distinctive wedge-shape
crystals, crystalline aggregate, & massive. |
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One perfect and some poor. |
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Varies between 3.26 to
3.36; Magnesioaxinite is 3.18 |
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Gamma at 1.68 |
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Biaxial negative; positive
if high in magnesium. |
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Usually strong trichroic
in reddish-brown, yellow-brown, olive green, yellow, & colorless; In
Luning, Nevada material: reddish-brown, deep brown, pale brown, or colorless;
In Sri Lankan material: reddish brown, deep violet, & colorless to
pale yellow. |
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In Franklin, New Jersey
material: SW=red; LW=inert; In magnesioaxinite from Tanzania: SW=dull red;
LW=orange-red. |
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broad lines at 4150, 4660,
& 4920; narrow line at 5120; possible lines at 4440 & 5320. |
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dissolves slowly in HF;
use HCl with caution. |
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avoid thermal shock; fusible
with intumescence (swelling) at 2.5 to 3.0. |
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is seldom flawless; has
color zones, feathers, & rare two-phase inclusions. |