Crystal Classics

29th March 2007

Exceptional Henmilite specimens from Japan

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)

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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