ETTRINGITE FROM THE KALAHARI DESERT AND YOUR SIDEWALK

Why Do You Want To Know?

A recent post on the Denver Spring Show was a generalized “here is a sampling of what I saw at the venue.” As stated in this post, I came home with only a few specimens, and one was a nicely colored group of hexagonal ettringite crystals [Ca₆Al₂(SO₄)₃(OH)₁₂-26H₂O].

Ettringite is an interesting mineral, a hydrous calcium aluminum sulfate, and is somewhat confusing to me. At the Type Locality in Rhineland-Palatinate, Germany, ettringite crystals are tiny, hexagonal, gemmy clear, and many have nice terminations with rhombohedral faces—think quartz look-a-likes. A few are even very thin, white fibers. Crystals at the TL are associated with metamorphosed limestone.

Talk to a construction/economic/chemical mineralogist and they will tell you that ettringite is a “hexacalcium aluminum trisulfate hydrate” that is formed in Portland cement as a result of the reaction of tricalcium aluminate  (C₃A) with calcium sulfate (CaSO₄): C₃A + 3 CaSO₄ → ettringite (Merlini and others, 2008). Did you ever wonder why freshly poured concrete gets warm (one of life’s persistent questions)?  The C₃A hydration is very exothermic and occurs quickly in the fresh concrete mix; the temperature increases with the fast progress of the hydration reaction. The workers “in the know” (those cement chemists) then add gypsum (CaSO₄·2H₂O) and other materials [perhaps heated (sintered) limestone, clay, fly ash. etc.] to the Portland cement to control the concrete setting. It seems like the gypsum allows  ettringite to coat the C₃A grains and slows down the hydration (Divet, 2000). WOW. I thought you simply added some sand and water to a bag of cement, mix, and presto with some time you get hard concrete!   

All of these ruminations on cement mixing bring back, from the far reaches of my mind, some unpleasant memories of a 7:30 am class on construction mineralogy (mainly taught to civil engineering students) or some such thing. Luckily, I was auditing the class and did not take the tests. That choice was a masterful stroke of genius on my part since I understood very little of what was going on, especially at 7:30 am. And finally, don’t be confused with the chemistry as those pesky cement chemists use their own chemical notations such as C=CaO and A=Al₂O₃! 

DON"T ASK ME HOW TO FORMAT WORD!!!!! 

Above: Ettringite crystals formed in concrete. I am guessing this is a SEM photomicrograph with the longest crystal ~ 40 microns. Credit Farnam’s Research Group at Drexel University, Drexel Advanced and Sustainable Infrastructure Materials Lab for the photo. Published in National Precast Concrete Association / Precast Magazines / Precast Inc. Magazine / 2018 – July-August

Crystals of tan to lemon-yellow ettringite/ The dark (black) material is some sort of manganese oxide, probably manganite [Mn³⁺O(OH)] while the chalky white mineral is most likely oyelite [Ca₁₀Si₈B₂O₂₉ · 12.5H₂O]. Width FOV top ~1.6 cm, middle ~1.7 cm, bottom ~6 mm

I need to unpack some of that material from my mind and toss it in the garbage. But hold on—as a life-long learner I now know why concrete heats up when poured, and the need to spray water on fresh concrete to retard evaporation and slow down the hydration and produce stronger concrete. That little bit of learning eases my rumbling mind!!

All of this chatter leads me back to ettringite from the Denver show. Many rockhounds have nice yellow to lemon-yellow ettringite crystals in their collection and would not recognize minerals from the Type Locality nor realize that their sidewalk probably has tiny ettringite crystals. Almost all collectable ettringite crystals come from the Kalahari Manganese Field in the northern Cape Province, South Africa, especially from the N'Chwaning Mines and some from the Wessels Mine. These mines were not really operational until the 1970s and 1980s; therefore, the great mineral specimens from the mines only appeared on the market in the last 50 years or so. MinDat noted that “apart from their ore producing significance [manganese], the N'Chwaning mines are notable and famous among the mineral collecting community for producing high-quality mineral specimens of rhodochrosite, manganite, ettringite, inesite, jouravskite and other minerals. The majority of the major mineral finds documented originate from the N'Chwaning II shaft [production started in 1981]… The manganese ores of the Kalahari Manganese Field are contained within sediments of the Hotazel Formation of the Griqualand West Sequence, a subdivision of the Proterozoic Transvaal Supergroup.” These late discoveries explain why my favorite mineral book, Encyclopedia of Minerals (Roberts and others, 1974), does not mention the Kalahari ettringite.

I “want to know” since it gives me a purpose in life. It makes my life worthwhile. It is what gets me up in the morning. It keeps me healthy, happy, and hopefully creative. It pumps up my mojo.

REFERENCES CITED

Divet, Loïc, 2000, State of knowledge on the possible causes of sulfate reactions internal to concrete: Bulletin de Liaison des Laboratoires des Ponts et Chaussées, Number 227.

Merlini, M., G. Artioli, T. Cerulli, F. Cella, and A. Bravo, 2008, Tricalcium aluminate hydration in additivated systems. A crystallographic study by SR-XRPD: Cement and Concrete Research, Elsevier vol. 38, no. 4. 

Roberts, W.L., G.R. Rapp, Jr., and J. Weber, 1974, Encyclopedia of Minerals: Van Nostrand Reinhold Company, New York.