Ettringite or Sturmanite?

I can’t tell for sure

Specimen is 24 mm vertically.

I got this specimen for $20 (a lot for me at the time) in 1987 because it was a beautiful and unusual yellow, had nicely formed crystals, and I had never heard of sturmanite. That’s not too surprising, as it had only been described as a new mineral in 1983 (Peacor and others, 1983, Sturmanite, a ferric iron, boron analogue of ettringite: The Canadian Mineralogist, 21 (4) 705-709).

Fast forward to the internet and the online database, MinDat. I quickly discovered that ettringite can look essentially identical to sturmanite and has been known much longer (named in 1874). Both occur at the N'Chwaning Mines in Cape Province, South Africa, where mine is from. Both can be associated with hausmannite (manganese oxide), as mine is. Sturmanite is Ca₆Fe₂(SO₄)_2.5[B(OH)₄](OH)₁₂·25H₂O and ettringite is Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O – differing mostly in containing iron vs. aluminum.

By most accounts, it’s impossible to discriminate sturmanite, ettringite, and the closely related charlesite (Ca₆(Al,Si)₂(SO₄)₂[B(OH)₄](OH,O)₁₂ · 26H₂O) with certainty, even with analysis, because all the phases can coexist even within the same crystal. Given that, I don’t much care exactly what it is – I know its basic mineralogic makeup and it’s still really pretty. On the whole I suspect that my specimen is more likely to be ettringite than sturmanite despite the original label; many other examples labeled sturmanite showing the nice yellow crystals may likewise be ettringite, or mostly ettringite, or combinations.

Although those accessory minerals don’t contain manganese, the N’Chwaning Mines began to exploit manganese in 1972, with N’Chwaning II in 1981 and N’Chwaning III in 2006. They are in the Kalahari Manganese Field, the largest concentration of manganese in the world. From the collector point of view the Kalahari Manganese Field is probably most famous for its intensely red, well-formed rhodochrosite (manganese carbonate) crystals. My little scalenohedrons below are from Wessels mine at Hotazel, near the N’Chwaning and other manganese mines.

The rhodochrosite is secondary to original manganese silicates and oxides. The ore is intimately associated with banded iron formation and is of the same age (around 2,400 million years) – the time of the Great Oxygenation Event when iron precipitated out of the ocean as cyanobacteria produced oxygen in greater and greater amounts. The manganese may have precipitated in a similar way, but many questions about its origin remain, including why 50% (some estimates say 70%) of all the manganese in the world is in the South African manganese deposits.

South Africa’s huge manganese deposits may reflect an intriguing chapter in earth history. Dated at 2.4 billion years ago, these manganese-bearing rocks lie immediately above (and are therefore just younger than) sedimentary rocks laid down during the first known “Snowball Earth” episode 2.5 to 2.4 billion years ago, when the entire earth was encased in ice and snow. Joseph L. Kirschvink at California’s Jet Propulsion Lab and his colleagues suggest that a dramatic increase in free oxygen occurred once the earth and oceans lost their ice cover, and photosynthesizing cyanobacteria were able to produce oxygen.

The sudden bloom of life-generated oxygen would have reacted with both iron and manganese, stored in the anaerobic (oxygen-poor) oceans, with iron and manganese oxides precipitating and depositing on the early ocean floor. More conventional explanations for the Kalahari Manganese Field include emanations from distant volcanic centers and later hydrothermal alteration and deposition related to faulting, but the Kirschvink group’s ideas neatly tie the deposit to both earth’s physical history and the early development of life. (See also Tsikos and Moore, 1998, The Kalahari manganese field: an enigmatic association of iron and manganese: S. African J. Geol. 101 (4), 287-290; and Robbins and others, 2023, Manganese oxides, Earth surface oxygenation, and the rise of oxygenic photosynthesis: Earth-Science Reviews Volume 239.)

For an in-depth (one hour 20 minutes) look at the history of the Kalahari Manganese Field and its minerals by geologist Bruce Cairncross, check out the following YouTube video:

The United States is 100% dependent on imports for manganese, which is used to make steel. Primary import sources are Gabon (67% of imports) and South Africa (19%). Together South Africa (36%) and Gabon (23%) dominate world manganese production, with Australia (17%) in the #3 position.

The section above on manganese and Snowball Earth is from my book What Things Are Made Of, p. 205.