Exceptional Henmilite specimens from Japan29 Mar 2007 By Robin Henmilite
is a rare hydrated Calcium Copper Borate:
Ca2Cu[B(OH)4]2(OH)4
or empirical formula Ca2CuB2(OH)12
Henmilite crystals are a very attractive vivid violet blue colour and are mostly translucent, glowing ultraviolet when light shines through
them. Henmilite has vitreous lustre, a hardness of
2, a specific gravity of 2.52, and no cleavages. It is thought to be stable in
most environments. The crystal form is triclinic, and the crystals are normally in euhedral
pinacoidal crystals. Well formed crystals are rare, most are under 1mm in size
and generally Henmilite forms in anhedral masses.
Henmilite is thought to be found at only one location (therefore also its type location) Fuka
mine, Bitchu-cho (Bicchu-cho), Takahashi, Okayama
Prefecture, Chugoku Region, Honshu Island, Japan. The Fuka mine is
approximately 40 km northeast of Okayama
city. It is an underground mine for very pure calcite which was used by a
toothpaste company. The area is well known for the occurrence of skarn minerals
with interesting species occuring in high-temperature, boron-bearing, very low-Fe
calc-silicate skarns between the Calcite orebody and igneous intrusives. Collecting
at the mine is now prohibited. (ref. mindat.org)
Bitchu - Matsuyama Castle
Courtesy www.jcastle.info
The Henmilite crystals are found in small cavities in Borate-rich
veins cutting contact-metasomatized marble or limestone. It is often associated
with Calcite and rare Borate minerals such as Pentahydroborite and Olshanskyite
(both Calcium Borates). The matrix is normally white providing a great backdrop to the vivid blue of the Henmilite and is light in weight and porous with a vuggy texture.
The specimens we have posted were all found (to our knowledge) in a single
pocket, this was the best find of Henmilite crystals with very large
perfect crystals aesthetically placed on white matrix. They were found only a few years ago just below collecting became prohibited. The crystal all have
very good colour and luster and are very big in size for Henmilite measuring
between 1mm to over 5mm. The Henmilite crystals tend to be scattered over the
matrix in exposed vugs, and/or partially intergrown with the matrix. 
The matrix of these specimens is Calcite rich, generally white and finely
porous, with little weight. It often has a mamilliary form, particularly within
vugs, or an almost stalactitic with formations like you see in an underground cave. Often there are fine clear crystals
<1mm in size growing from the surface of the matrix. We have described our
matrix as Calcite, but we assume (but have not identified) rare Borate minerals
such as Pentahydroborite and Olshanskyite are probably present.
We hope you enjoy these exceptional specimens for their rarity as a mineral species, the perfect large crystals, and the quality of the colour and lustre.
References:
- Henmilite, Ca2Cu(OH)4[B(OH)4]2, a new mineral from Fuka,
Okayama Prefecture, Japan – American Mineralogist, Vol 71, pages 1234-1239,
1986.
- http://www.mindat.org/show.php?id=1866
- http://www.webmineral.com/data/Henmilite.shtml
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