Sunday, November 25, 2012

DIOPTASE vs. DIOPSIDE


I cannot pretend to be partial about the colours, I rejoice with the brilliant ones…
Like Winston Churchill quoted above, I am quite partial to bright colors found in some minerals, but this trait is likely the norm among most collectors.  And, I am especially fond of green minerals—peridot, some garnets, some tourmaline, malachite, emeralds, and several others.  So, in perusing my collection, I came across green examples of dioptase and diopside and decided to further investigate.  In fact, I knew little about either mineral except several decades ago I attended a summer geology field camp and was shown chrome diopside from diatreme rocks in what is now the State Line Diamond District of Colorado and Wyoming.   And, I probably thought the two minerals were somehow related due to their color and their “diop…”.   However, I have found that diopside is named for the Greek prefix di, or two, while dioptase is for the Greek dio, or through.  And, they are not related.

BLOCKY, DARK GREEN DIOPTASE CRYSTALS FROM NAMIBIA.  WIDTH ~1.0 CM.

Dioptase is a hydrated copper silicate (CuSiO3-H2O) whose emerald green color is due to the copper cation.  The mineral is translucent to transparent, the individual crystals are mostly six sided and capped by a rhombohedon.  Although fairly soft at 5 on the Mohs scale, some cutters facet the gemmy crystals but must install the finished stone in a pendant—it is too fragile for a ring or bracelet.
NICE, SEMI-GEMMY CRYSTALS OF DIOPTASE FROM ALTYN-TYUBE, KAZAKHSTAN.  WIDTH OF PHOTO ~4.0 CM.
Dioptase is a secondary mineral that is found in oxidized zones associated with hydrothermal replacement in copper sulfide deposits.  One specimen is my collection came from the Christoph Mine, Kaokoveld Plateau, Kunene Region, Namibia.  The best I can determine is that dioptase, and several other oxidized secondary minerals, are found in limestones associated with the Otavi Group (late Proterozoic [Precambrian] ~600-800 Ma).  The mine, and adjacent area, is “famous” for producing dioptase crystals.

MASS OF VUGGY, MOSTLY CERUSSITE, CRYSTALS FROM ARIZONA.  SEE PHOTOMICROGRAPH BELOW.  WIDTH ~ 2.1 CM.  NICE, SEMI-GEMMY CRYSTALS OF DIOPTASE FROM ALTYN-TYUBE, KAZAKHSTAN.  WIDTH OF PHOTO ~4.0 CM.
The second specimen is from the famed Mammoth-St. Anthony Mine (Tiger) located about 50 miles northeast of Tucson in Pinal County.  A variety of metals, including gold, has been mined from the shafts but all activity ceased many years ago.  “Mineralization in this district is a series of veins within shear zones…Their gangue consists of brecciated country rock, cemented and replaced with quartz and calcite together with some barite and fluorite” (www.mindat.com).  My particular specimen is a mass of vuggy cerussite crystals (clear and gemmy) with sparse tiny crystals of green dioptase, and tiny arborescent “growths” of the mineral.  Very small partial crystals of wulfenite may also be present.

PHOTOMICROGRAPH OF GEMMY AND CLEAR CERUSSITE CRYSTALS WITH TINY GREEN CRYSTALS OF DIOPTASE.  WIDTH OF PHOTO ~2.45 MM.
Diopside is a calcium magnesium silicate (CaMgSi2O6) and a member of the pyroxene family. It is the magnesium-rich end member of a solid solution series with augite and hedenbergite (iron-rich).  Prismatic crystals are rather square in cross-section.  Most “common” diopside is green or sometime colorless/white while chrome diopside is a nice gemmy stone with chromium imparting a deep green color.  At times rutile is included and cat’s eye or star diopside (four-rayed) is produced.  Most specimens of diopside have a vitreous luster and are translucent to transparent with a hardness of 5-6 on Mohs scale.  Most gem cutters prefer to facet chrome diopside although some “common” diopside is cut, especially if the crystals are gemmy and clear. 


Diopside may occur in a variety of environments-- in hornfels associated with regional and contact metamorphic zones, in kimberlites associated with diatremes, in metamorphic gneiss and schist, and in skarns.
GEMMY CRYSTALS (PROBABLY CHROME-RICH) FROM MINAS GERAIS, BRAZIL  WIDTH ABOUT 3.8 CM.
I have two diopside specimens, one from Minas Gerais, Brazil (Aracuai [Arrassuai], Jequitinhonha Valley) where very gemmy crystals (probably chrome-rich) are associated with quartz.  The second specimen includes several (some quite large), non-gemmy blocky crystals from the York River Skarn, Hastings Co., Ontario.  


LARGE DIOPSIDE CRYSTALS FROM HASTINGS. CO., ONTARIO.  WIDTH OF TOP CRYSTAL ~2.4 CM.
So, although diopside and dioptase seem like they might be closely related, they are really “far apart”.  One is a hydrated secondary copper mineral while the other is a magnesium-iron mineral usually found in metamorphic rocks (or sometimes peridotites).   
Churchill finished by his thoughts about color by saying, and I am genuinely sorry for the poor browns!


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