MANGANESE: STURMANITE FROM THE KALAHARI

A few months ago I was working on a post describing kutnorhorite, a carbonate that was not, at that time, in my vocabulary [Ca(Mn,Mg,Fe)(CO₃)₂]. The mineral had been collected in the Kalahari Manganese Field, Northern Cape Province, South Africa, a locality I vaguely recognized.  However, in reading additional articles about the field I became duly impressed for it is the world’s most prolific producer of manganese. The deposits are in the Hotazel Formation of Proterozoic age (Precambrian) and are the largest land-based (deep sea deposits are larger but nearly impossible to mine) sedimentary manganese deposits in the world, perhaps covering ~425 sq. miles. The ore has been subjected to both hydrothermal alteration (temperatures up to 450°C) and to metamorphism. The origin of the giant manganese deposits has been debated for many years but remain controversial: “Proposed models cover a diverse spectrum of genetic processes, from large-scale epigenetic replacement mechanisms, to submarine volcanogenic-exhalative activity, to purely chemical sedimentation whereby the influence of volcanism is of reduced significance” (Tsikos and Moore, 2006).

Another impressive fact about the Kalahari is that the mining region has produced some of the world’s finest rhodochrosite and hematite, as well as the finest ettringite, sturmanite, olmiite, sugilite, inesite, hausmannite, and thaumasite. Two of these mines (Wessels and N’Chwaning II) are together type localities for 19 species, including diegogattaite, wesselsite, lavinskyite, manganvesuvianite, marshallsussmanite, and sturmanite (www.mineralspecies.com).  Impressive, correct?

I did not recognize most of those mineral names except sugilite and sturmanite, the former for the lapidary qualities and the latter for the nice crystals in a specimen purchased a few years ago.

 

Photomicrograph of sturmanite crystals.  Note elongated hexagonal and terminated crystal with a length of ~7 mm.

Sturmanite is a hydrous calcium aluminum iron sulfate with some boron [B(OH)₄] ions substituting for some of the sulfate ions: [Ca₆(Fe^3+,Al,Mn³⁺)₂(SO₄)₂[B(OH)₄](OH)₁₂-25H₂O]. It often occurs in sharply terminated crystals (dipyramidal tabular to elongated hexagonal crystals according to MinDat) that are nice looking amber to bright or dark yellow in color. Crystals are transparent to translucent, mostly vitreous, and fairly soft at 2.5 (Mohs). Many are gemmy.  Outside of the South Africa mines MinDat noted only one other occurrence—in the Northern Caucasus Region in Russia.  It is a metamorphic mineral found in secondary cavities in stratified manganese deposits.

 

Photomicrograph sturmanite crystals. Note nice termination of gemmy crystal far left.  Width FOV ~1.3 cm.

So, I was all set to go with this description when I was browsing one of my favorite web sites, www.dakotamatrix.com, and noted a specimen of sturmanite was listed “for sale” under the July 3 “Mixed Minerals” banner.  Out of curiosity I clicked and was rewarded with the following information gleaned by Tom: I learned by Malcolm Southwood's blog that it is virtually impossible to distinguish between members of the Ettringite group…Furthermore, most of the minerals in this five amber group are very hydrated so decomposition occurs and there is also compositional zoning… Crystal habit and color alone will not do it.

I could not locate the Southwood Blog but the Ettringite Group consists of 12 (www.mindat.com) hydrated calcium sulfate hydroxide minerals with a common trigonal structure (www.galleries.com/galleries).  But I still would like to call my specimen sturmanite. 

REFERENCES CITED

Tsikos, H. and J.M. Moore, 2006, The chemostratigraphy of a Paleoproterozoic MnF- BIF succession -the Voelwater Subgroup of the Transvaal Supergroup in Griqualand West, South Africa:  South African Journal of Geology, v. 109.

CORNETITE: BLUE COPPER PHOSPHATE

I am a sucker when it comes to purchasing blue, red or green minerals if the price is reasonable (cheap), and especially if they are phosphates, arsenates, or vanadates.  Two thoughts here: 1) I noted in a previous blog that I purchase minerals, not for an office display nor a case at the local rock show, but to learn as much as possible about the species and then “write it up” for this blog or a club newsletter.  One of my goals in life is to continue, “till the end,” to be a life-long learner.  So, spending something in four figures would put a crimp on purchasing 200 other specimens---my learning curve would bottom out.  Strange---perhaps, but this life–long learning is a passion and helps keep me intellectually alive; and 2)  the arsenates contain the AsO₄ radical and are usually grouped together with the phosphates (PO₄ radical) and the vanadates (VO₄ radical) since the three radicals are similar in size and commonly substitute for each other (and change the mineral).  The total minerals in these three groups number in the hundreds and are second only to the silicates in total number. Many are colorful; however, most members of the group are rare and only a few are common (such as the phosphatic apatite minerals).

While rummaging around down in Tucson last February at some small out-of-the way venue I noticed something “blue” that really caught my eye.  Snagging the specimen out from under a few other boxes I became really interested: 1) the specimen was priced under $5; 2) the specimen was deaccessioned from the “Mineralogical Museum, New Mexico Bureau of Geology & Mineral Resources, New Mexico Tech.”  Labels like this on a mineral specimen are an added bonus.  I also picked up a second specimen, via mail order, from Sauktown Sales.

OK, the blue mineral is cornetite, a copper phosphate hydroxide—Cu₃(PO₄)(OH)₃—that is relatively rare in the record.  It occurs as a secondary mineral of the oxidized zone in a few hydrothermal copper deposits.  Cornetite is not as common as the other basic copper phosphates libethenite [Cu₂PO₄(OH)] and pseudomalachite [Cu₅(PO₄)₂(OH)₄]---see Blog Posting January 11, 2015. I have been unable to find a reference that would lead me to understand why cornetite is such an uncommon mineral!  What are the conditions necessary for the formation of cornetite?  Another day, another question awaiting an answer!

 

Photomicrograph with bright blue equant crystals and long prismatic crystals in large specimen. Long dimension of largest cluster ~3.5 mm.

Photomicrograph of scattered crystals of cornetite.  Largest cluster is noted in close up above.

Clusters of cornetite rosettes forming a crust. Width FOV ~1.5 cm.

Cornetite is an attractive collectable mineral due to its bright blue to bluish-green color on tiny vitreous crystals.  The crystals are microscopic prismatic, some are equant, and are one to two millimeters or less in length and sometimes rounded. They often cluster together in radial aggregates or rosettes and sometimes form a crust.  They have a hardness of ~4.5 (Mohs) and range from transparent too translucent.  Crystals may be confused with azurite; however, cornetite is not a carbonate, as is azurite, and will not effervesce in hydrochloric acid (although it is soluble in cold acid).

Cornetite is an uncommon mineral and is only known from a few localities in the United States (Colorado, Arizona, New Mexico and Nevada with several questionable occurrences), Australia, Chile and a few European countries. It is best known from the Democratic Republic of Congo (Zaire), the type locality (L'Etoile du Congo Mine [Star of the Congo Mine; Kalukuluku Mine]) and the site of my specimens. The Etoile orebody lies within the copper belt that stretches from Luanshya in Zambia to Kolwezi in the Democratic Republic of Congo. Shalina Resources (2010) noted that as with many of the deposits in southern Katanga, Etoile is a stratiform copper-cobalt deposit. There is an enlarged oxide cap of about 160 ft that overlies an inclined stratiform sulfide deposit.  Although abandoned in the 1960s, it is my understanding that the mine went back on line after new owners (Shalina Resources) purchased the area in 2003.  The open pit mine processes supergene replacement minerals from the upper oxide zone but has plans to continue mining down into sulfide deposits.  Etoile is rich in cobalt (paying half the bills) and I presume they are mining spherocobaltite [CoCO₃] and Heterogenite [CoO(OH)], minerals listed by MinDat.  There are a variety of copper oxides in the enriched zone included malachite [Cu₂(CO₃)(OH)₂] and tenorite [CuO].  However, I have been unable to locate information about the minerals available for mining in the underlying sulfide zone. 

So, cornetite is another blue copper phosphate that is a nice addition to my modest collection.

REFERENCES CITED

shalinaresources.com/geology.aspx (2010)