My last posting described the major economic discovery of REE/Critical Minerals located in a fairly nondescript (surficial description) area of southeastern Nebraska called Elk Creek. At this locality, ~600 feet of Pennsylvanian marine rocks, topped with varying small amounts of glacial debris, cover some very complex subcrops of Precambrian (Proterozoic) rocks. Most geologists working with Phanerozoic (post- Precambrian) rocks of the Plains and Midwest simply refer to these subcrops as the “basement.” I certainly did during my stints of teaching Historical Geology. Most of the class time was spent studying fossiliferous sedimentary rocks unless those pesky mountains were being emplaced (life became easier with the emerging knowledge of plate tectonics). In places like Nebraska and Kansas, surficial outcrops of Precambrian rocks were essentially unknown. The good news was that field trips to the Colorado Precambrian outcrops were always enjoyed.
My friends in the field of geophysics seemed to be the only people who really enjoyed the “basement.” They could look at a page of squiggly lines and come up with ideas and interpretations about anomalies, faults, basins, etc. I believed these were secret codes that geophysicists used to communicate with each other and tease the paleontologists and soft rockers. I admit that interpretations of squiggly lines easily confuse me; therefore, I rely on my friends, and professional journals, to locate interpretations.
Several decades ago, I was a student in the graduate program at the University of South Dakota in Vermillion. Today USD is a well-known university, especially in the Plains and Midwest, due to their top-notch academics, and their winning athletic teams. During my two-year tenure in Vermillion USD was sort of lost among the schools of the Big 10 and Big 8. The academics were good and the athletic teams excellent/OK; however, in those days before widespread TV coverage the “Dakotas” were cold and somewhere near Canada and maybe just a few miles from the Arctic Circle. While traveling out of the state, especially to my home state of Kansas, I got many opportunities to answer questions about the small-town kid who disappeared from home. Normally, questions centered around, “where are you working”, or “where did you say you were going to school.” My answers usually brought blank stares and finally they recovered with “is that near the Black Hills.” But I enjoyed my stay, learned much, met many “really nice” people, and always looked forward to field trips to the Hills, about 400 miles to the west. USD was closer to my home in Kansas than to the Hills (but a lot less interesting). At any rate, I took a course in geophysics trying to help calm my fear of squiggly lines—no such luck. But one thing I do remember were assignments trying to interpret those early gravity and magnetic maps. One assignment was for students (group project) to interpretate a magnetic map with an anomaly situated in a sedimentary rock section out near the Hills. Oh, the students dreamed up a variety of scenarios except the easy one. Turns out this anomaly was centered on a “gravel pit” of Pleistocene age where many high-content iron minerals, like biotite, tourmaline, magnetite and hematite, accumulated after their weathering from igneous and metamorphic rocks and stream transportation from the Hills. An embarrassing situation for ?smart students. Oh well, we learned.
The Department, or the SD Geological Survey, also had a magnetometer that was cumbersome, old, cranky, and tough to haul around and use, but we “sort of” learned the principles of use. I was then amazed to find out that a senior South Dakota Survey geologist had published, in 1962, the Magnetometer map of southeastern South Dakota. Our class discussions then moved to this map and the source of magnetic highs in the far southeast corner of the state. The only possibility that we could support was fault movement or doming of the Precambrian basement rocks. Close but no cookie! My fellow group members were also confused by gravity and magnetic maps! This map revealed the presence, not of gravel pits nor outcrops of faulted igneous rocks, but of a band of mafic intrusive igneous rocks subcropping in Union County, next door to Sioux County, Iowa (and very near Vermillion).
So, my stay in Vermillion was 1965-67, the SD map was published in 1962, and somewhere in that period of time Iowa had also identified several of these magnetic anomalies since an exploratory drilling program and aeromagnetic surveys had been conducted by the New Jersey Zinc Company starting somewhere around 1963. The company was looking for possible iron ore of economic significance when in 1966 they drilled through about a thousand feet of glacial debris and sedimentary rocks and then cored, with a diamond bit, as much as 500 feet of Precambrian igneous/metamorphic rocks. Since the drill site was near the small town of Matlock, Iowa, one often sees the term Matlock Drill Core (see MinDat). In fact, the Company drilled 12 different wells into and around the anomaly.
In addition, MinDat lists 30 known minerals from these cores. One of the cores came from an ultrabasic serpentinite created by metamorphism of an igneous rock originally composed of olivine and pyroxene that was altered (minerals replaced) by serpentine, brucite, and magnesite with veinlets in the serpentine filled with dolomite, calcite, pyrite, brucite, magnesite, and other unknowns (Cordua, 1990). Two geologists from the New Jersey Zinc Company, in examining the core, noted “a material giving an unidentified X-ray diffraction pattern …coming from a bluish green, translucent, platy, soapy mineral…a previously undescribed hydrous magnesium hydroxide-ferric oxychloride” they named Iowaite (Mg6Fe3+2(OH)16Cl2 · 4H2O) to honor the State of Iowa (Kohls and Rodda, 1967).
But there is another story associated with the naming of this new mineral, and that is the extent of these magnetic highs and the composition of these anomalies, and the reason for their existence. So now the story switches over to a great paper by Windom, Seifert, and Anderson (1991). What I find amazing, with regards to this work, is reading about the new amount of information that came forth in the previous 25 years (1966-1991).
I believe that the original SD magnetic map was created from information provided by lugging that old magnetometer (Askania Vertical Ground Magnetometer—maybe) over county roads and pasture lanes to acquire readings about every five miles. My confidence in this call is based on the 1961 land survey of Tood and Mellette Counties by Survey geologist Bruno Petch along the Nebraska-South Dakota state line in the south-central part of the state. I am less familiar with Iowa so I remain uncertain if hand magnetometers were used in northwest Iowa or if aeromagnetic surveys were the choice of the day.
At any rate, after completion of drilling the 12 wells, and core examination at Matlock, information delineated a very complex assortment of layered igneous and metamorphic rocks that were now tilted and dipping to the northwest about 25 degrees. Their total stratigraphic thickness may exceed ~8000 feet (Windom and others, 1991) and are Precambrian Archean (2890 +- 90 Ma) in age (Van Schmus and Wallin, 1991). These units were given the name of Otter Creek Layered Igneous Complex and Otter Creek Magnetic Anomaly.
Additional magnetic surveys (probably aeromagnetic) has shown the Otter Creek Anomaly is related to a string of nearby magnetic anomalies trending southwest-northeast from South Dakota through Iowa and into southwestern Minnesota.
This location map, courtesy of Windom and others (1991), shows the relationship of the Otter Creek Magnetic Anomaly (*) to the other known associated magnetic anomalies associated with the southern boundary of the Superior Province.
These anomalies lie parallel to, but northwest of, the Spirit Lake Trend (SLT) (Windom and others, 1991). The SLT has been described as the boundary between Precambrian Archean rocks (2.5 Ga or older) of the Superior Province and the Precambrian Proterozoic rocks of the Penokean Province (2.5 Ga to 600 Ma) (Windom and others, 1991). Van Schmus and Wallin (1991), as noted above, defined an Archean age (2.39+- Ga) to the Otter Creek Layered Complex just northwest of the SLT, and dates of 2.5 to 3.0 Ga throughout the Superior Province. Southeast of the SLT, core samples from Nebraska and South Dakota are from ~1.76 Ga to ~1.80 Ga, or Proterozoic in age. The contact between the terranes seem sharp.
Map from Whitmeyer and Karlstrom (2007) showing location of Yavapai terrain. Black butterfly is hovering over the tall grass prairie at Matlock, Iowa, along the Spirit Lake Trend. Compare with previous map.
Whitmeyer and Karlstrom (2007) and Van Schmus and others (1991, 2007) also believed the Penokean crustal rocks are limited to the central and northern parts of Wisconsin, Minnesota, and Michigan, They noted the Yavapai Province crustal rocks continue from Arizona eastward through Colorado south of the Cheyenne Belt, Nebraska, the mid-continent region, eastward further into Ontario and then further east into the protolith of the Grenville Province. However, dates associated with the Yavapai Orogen/Province do overlap with dates of the Penokean crustal rocks; interestingly, these two provinces are interpreted in terms of subduction flip from south dipping in the Penokean orogeny to north dipping along the southern border of the Superior Province/Laurentia. In addition, Windom and others (1991) used the term Central Plains Province to describe the eastward extensions of Colorado Proterozoic rocks into the mid-continent region. In today’s language these rocks are now part of the Yavapai Province (Whitmeyer and Karlstrom, 2007). The Yavapai Orogeny is now defined in terms of a long-lived convergent plate margin orogen along a southward-growing Laurentia. Most of this new crust is the result of a series of separate oceanic arcs that developed diachronously outboard of Laurentia and became welded together and to Laurentia (Whitmeyer and Karlstrom, 2007).
But back to iowaite and Iowa. The mineral from the Matlock core was never abundant and any/all studies on iowaite had to come from tiny specimens retrieved from the drill core. In fact, Cordua (1990) stated, “iowaite has, to my knowledge, only been found in this one drill core in this one spot.” That all changed in 1983 when Jon Gliddon, the Manager of Mining at the large Palabora Mine, Limpopo, South Africa, discovered a mineral he provisionally identified as pyroaurite (Mg6Fe3+2(OH)16[CO3] · 4H2O). To confirm his identification, Gliddon sent a sample to Richard Braithwaite, a well-known mineralogist at the University of Manchester in the UK. Using microprobe, carbon analyses, thermal analyses, w-ray diffraction, mass spectrometer and optical studies Braitwaite and his colleagues (1994) stated “this new material have shown that it is indeed similar to pyroaurite, but with chloride taking the place of most of the interlayer carbonate in the latter, and despite some differences in analyses and physical properties, seems to be identical with iowaite” (Mg6Fe3+2(OH)16Cl2 · 4H2O). And, it turns out, that the Palabora specimens are far superior to the Matlock Core specimens in purity, crystal distinction, size and greater availability. The Palabora specimens are well crystalized and are similar in habit to their relatives in the Chlorite Group. For a much better description of iowaite see: Cairncross (2018), Braithwaite and others (1994), Gliddon and Braithwaite (1991), and Southwood and Cairncross (2017). In addition, MinDat (assessed 16 Oct 25) noted that iowaite “is bluish green, becoming pale green with a rusty red tint on exposure to air (alteration to pyroaurite).”
Since the cat came out of the bag in Palabora, iowaite has shown up in Australia, three Canadian provinces, China, France, Oman, Poland, four localities in Russia, Spain, Uzbekistan, Sterling Mine in New Jersey, and deep ocean sediments in both the Atlantic and Pacific Oceans.
However, the iowaite story continues. I certainly did not have the slightest idea about these fascinating stories about iowaite until three years ago. I had heard of iowaite since minerals named after states are rare and at one time, I wrote a little story about coloradoite (a mercury telluride) and discovered the other three minerals. I knew about the Matlock Drill Core but do not remember why it was stuck into the back recesses of my mind. I tend to gravitate to strange and weird minerals and so when I saw a specimen of iowaite for sale in Denver 2022, I nabbed it. After the purchase I grabbed a coffee, rested my ole body, pulled out my phone and dialed up MinDat to examine iowaite. Wow, the specimen I purchased certainly did not resemble, or even seem related to, any mineral found in Iowa. What I had purchased was a nice specimen of chromium-bearing iowaite (Mg6(Fe3+,Cr3+)2(OH)16Cl2·4H2O ) where there is significant replacement of Fe3+ by Cr3+ that perhaps leads to a transition to woodallite (Mg6Cr2(OH)16Cl2 · 4H2O). This colorful variety of iowaite is collected from a single locality in an ultrabasic massif in the Altai Republic, Russia, somewhere in southern Siberia! Specimens of this variety are beautiful purple (of various shades) that is platy with a greasy feel. Unfortunately, I can locate very littles information about the discovery, and more importunately, the source of the chromium. Also, I assumed that such a seemly rare mineral (only the single locality I think) would be “pricey” on the mineral market. However, I noticed that on Etsy and Ebay that prices were quite modest, even ”cheap”, with thumbnails starting in the single digits. Digging a little deeper in MinDat I found that at the main collecting site, chromium minerals of the hydrotalcite group [including iowaite], are confined to linear zones in serpentinites, sometimes stretching for tens of meters, are represented by massive fine-scaled aggregates of purple/lilac color in different shades — from light pink-lilac to bright deep violet-lilac. They form lenses and nests up to 30 cm in diameter, as well as veins in chrysotile-lizardite serpentinites. They also form pseudomorphs along rounded chrome spinel grains in serpentinite. So, there seems to be a good source of material but getting it collected and out of Siberia might be difficult in today’s world? Presumably this information in MinDat is from a Russian publication, unavailable here in the Village Library in Holmen, WI.
Purple resinous mass of chromium-bearing iowaite with inclusions of greenish yellow/brown serpentine. Some of the lighter colored, lilac shade, may be closely related stitchtite. Width of specimen 2 cm.
This story of iowaite, at the time of discovery, seemly would restrict the rather non-descript mineral to the single drill core brought up from a serpentine-rich metamorphic rock about 1500 feet below the surface of a tall grass prairie in rural northeast Iowa near Matlock (population 74). Little additional work was completed on the mineral due to a lack of material in the core. But the 12 different drill cores in the area provided radiometric dates for the basement rock and helped build the foundation for mapping the Archean and Proterozoic boundaries.
So, iowaite was sort of moved to the back burner until an observant mine manager in South Africa sent samples of a mineral to a mineralogist in the UK who might have said, “wow this sample is the same as long forgotten iowaite from the colonies.” The sample from the Palabora opened up a wealth of information about iowaite and soon new localities were popping up across several continents. And somewhere in southern Siberia Russia miners/mineralogists opened a seam that would provide chromium-rich iowaite to collectors and researchers around the world. And now, in this century, synthetic iowaite had been cooked up in a chemical lab and today some deep hunting on the WEB will locate a plethora of research articles searching for industrial uses of, no kidding, iowaite. For example, see Molecules. 2021 May 20;26(10):3052. doi: 10.3390/molecules26103052: Synthetic Iowaite Can Effectively Remove Inorganic Ar.
Who woulda thought??
REFERENCES CITED
Browning, S.A. and K.E. Karlstrom, 1990, Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States: Geology (USA), vol. 18, no. 12.
Braithwaite, R.S.W. and J.P. Gliddon, 1991, Zeolites and associated minerals from the Palabora Mine, Transvaal [South Africa]: Mineralogical Record, vol. 22, no. 4.
Browning, S.A. and K.E. Karlstrom, 1990, Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States: Geology (USA), vol. 18, no. 12.
Cairncross, B., 2018, Iowaite, Sioux County, Iowa: Rocks and Minerals, vol. 93, no. 3.
Cordua, W.S., 1990, A mineral named for Iowa: Leaverite News, v. 15, no. 8, p. 2.
Kohls, D. W. and J.L. Rodda, 1967, Iowaite, a new hydrous magnesium hydroxide ferric oxychloride from the Precambrian of Iowa: The American Mineralogist, vol. 52, nos. 9 and 10.
Southwood, M. and B. Cairncross, 2017, The minerals of Palabora, Limpopo Province, South Africa: Rocks and Minerals, vol. 92, no. 5.
Van Schmus, W.D. and E.T. Wallin, 1991, Studies of the Precambrian Geology of Iowa: Part 3. Geochronologic data for the Matlock drill holes: Journal of the Iowa Academy of Science, vol.98, no. 4.
Van Schmus, W., D. Schneider, D. Holm, S. Dodson, S. and B. Nelson, 2007, New insights into the southern margin of the Archean–Proterozoic boundary in the North-Central United States based on U–Pb, Sm–Nd, and Ar–Ar geochronology: Precambrian Research, vol. 157, issues 1-4.
Windom, K.E., K.E. Seifert, and R.R. Anderson, 1991, Studies of the Precambrian geology of Iowa: Part 1. The Otter Creek layered igneous complex. Journal of the Iowa Academy of Science, vol. 98, no. 4.
Whitmeyer, S. and Karlstrom, K. E., 2007, Tectonic model for the Proterozoic growth of North America; Geosphere vol. 3, no 4.