Tuesday, April 2, 2013


The state of South Dakota has always occupied a place in my heart since graduating from the “U” in the late 1960’s.  As a native Kansan I was excited about traveling north to a different university and seeing new and exciting localities.  After an early fall field trip to the Black Hills, I was hooked on the state---forever.  I have returned to South Dakota many times since those halcyon days of my youth and always had my eyes to the ground looking for new specimens.  In addition, while attending rock/mineral shows and/or visiting rock shops I am on the lookout for specimens labeled as being collected from South Dakota.

At a recent spring rock show in Apache Junction, Arizona, I came across a dealer with a number of specimens acquired at a liquidation sale of some sort.  Much to my surprise, and excitement, one specimen was labeled “Arrojadite, Nickel Plate Mine, Pennington  Co., SD.  Collector: Bud Ehrle, 1960’s”, and priced for $3.  I snatched it up although not certain if I could differentiate between arrojadite, Arrowsmith, or Arrowhead Stadium! 

After returning to my library and computer to search the references and try to find out if arrojadite was even a valid mineral name, I begin to wonder what sort of a strange specimen I had acquired.  Turns out that arrojadite is an exceeding complex anhydrous phosphate with a “long” chemical formula: KNa4Ca(Mn,Fe)14Al(PO4)12(OH)2, or at least something close.  That caveat is here because the latest work on the arrojadite, at least that I could locate (Camara and others, 2006), indicates the mineral is in a solid solution series with three end members: Arrojadite (KNa) [potassium-sodium aluminum phosphate], Arrojadite (SrFe) [strontium-iron aluminum phosphate], and Dickinsonite (KMnNa) [potassium-manganese-sodium aluminum phosphate].  However, www.MinDat.org noted there are many other “in-between” series members with various combinations of barium, lead, potassium, sodium, iron, strontium, and manganese. I certainly agree with Huminicki and Hawthorne (2002) when they stated arrojadites are infernally complex structures…and Camara and others (2006) who noted there is no doubt that the structure and crystal chemistry of arrojadites, [are] perhaps among the most complex found so far.  The good news is that mineralogists do understand that the South Dakota Nickel Plate Mine specimens are the arrojadite (KFe) variety with a green to green-yellow color, a hardness of ~5 (Mohs), a conchoidal fracture, and a rather vitreous luster. 


Any member of the arrojadite group is rare in the mineral record.  In western North America there are minor occurrences in Canada (Yukon), Arizona, and South Dakota.  In the Black Hills www.MinDat.org noted arrojadite has been found in the Victory Mine (Custer County), Big Chief Mine, Etta Mine, White Cap Mine, and Nickel Plate Mine (Pennington County).  The latter mine, a one-time tin (cassiterite)- producing project, is the “type locality” for arrojadite (KFe) from a pegmatite near (east) of the Keystone cemetery. Dickinsonite (arrojadite group) and a variety of other minerals are also known from the mine.  I am hoping on my next visit to the Hills that my friends in South Dakota will guide me to the Nickel Plate Mine claim.  In the meantime, I have acquired a nice (but small), cheap ($3) specimen of a rare phosphate mineral. 

Cámara, F., R. Oberti, C. Chopin, and O. Medenbach, 2006, The arrojadite enigma: I. A new formula and a new model for the arrojadite structure. American Mineralogist, 91, 1249-1259.

Huminicki, D.M.C. and F.C. Hawthorne, 2002, The crystal chemistry of the phosphate minerals, in M.L. Kohn, J. Rakovan, and J.M. Hughes, Eds., Phosphates: geochemical, geobiological, and materials importance: Reviews in Mineralogy and Geochemistry, Mineralogical Society of America, Chantilly, Virginia.


Kansas is a state with abundant fossil and mineral resources, but with few specimens that rockhounds would consider gemstones or even semi-precious gemstones.  However, there are some very nice cabinet specimens available for most collectors.  The problem usually associated with collecting in Kansas is that virtually all land is privately owned and/or state land is off-limits to collecting---so, ask permission before trespassing. Minerals in the eastern part of the state were noted in the Blog Posting of March 23, 2013.

Generalized geologic map of Kansas.  The eastern one third of the state has bedrock of Mississippian, Pennsylvanian and Permian age (blues and purples). Outcrops of Cretaceous rocks (green) and Cenozoic (yellow, tan and brown) dominate in the western section.  Map from Kansas Geological Survey. 
The Cretaceous rocks cropping out in the center and western part of the state offer numerous mineral collecting localities, many more than the average rockhound is aware. Near, and at, Kanopolis Reservoir (west of Salina but near I-70, Exit 238) are exposures of gray-black shales of the Kiowa Formation.  Unique specimens of Turritella gastropods replaced by pyrite/marcasite are scattered along the shoreline of the reservoir as are nuggets of marcasite.  In other Cretaceous black shales the pyrite/marcasite is the fossilization agent in cephalopods such as Scaphites (Blue Hill Shale) and Baculites (Pierre Shale).   In addition to the Kanopolis gastropods, strange nests of fibrous calcite, cone-in-cone, occur in beds and lenses in the Kiowa Formation.  Rockhounds should explore exposures along the dam face (if Corps of Engineers regulations allow such) as collecting both cone-in-cone and Turritella is “easy”.


Part of cone-in-cone structure that has broken away from the main mass.  Note circular pattern on top surface.  From the Cretaceous Kiowa Formation.
East of the Reservoir is the small community of Bavaria (along KS 140) where “barite roses” (barite-cemented sand grains) have been collected in road cuts from the Kiowa.  I have not found the “roses” in many decades (one remaining from a teenage excursion) ) and also have some small tabular crystals.

Small cluster of barite crystals collected from Kiowa Formation near Bavaria, Kansas.
Amber is quite rare in Kansas, as elsewhere, but has been reported from the Kiowa Formation at Kanopolis Reservoir (locality now under water).  A description and photos of the amber may be located in Blog Posting September 23, 2012.  Perhaps additional exploration would turn up new localities?
In many exposures of the Cretaceous dark shales, gypsum crystals (selenite) are scattered on the surface.  The gypsum is secondary and may form from the weathering of pyrite/marcasite, or enrichment by groundwater.  Some of these crystals are from five to seven inches in length and many are twinned.  Perhaps one of the easiest places to collect is from exposures of the Dakota Formation (Cretaceous) in the overflow spillway at Wilson Reservoir (Russell County, north of Exit 206, I-70).  

Other Cretaceous black shales, especially the Blue Hill Shale Member of the Carlile Shale, often contain concretions that collectors term septarians.  When I was teaching in Kansas, numerous people showed up in my office lugging along their “fossil turtles”.  It was my duty to bust the balloon and inform them their rock was a septarian concretion.  I tried to encourage them to closely examine the concretion and notice that the internal cracks in the concretions are secondarily filled with calcite.  They actually make nice collectable specimens but nab a small one as some are larger than a motorcycle!   Rockhounds should consult a geologic map of Kansas and pinpoint outcrops, especially around Cedar Bluff Reservoir south of I-70 (Exit 135), and prospect.  In addition, some concretions in the Pierre Shale of Wallace County (far western Kansas) contain crystals of barite; however, most of these specimens are located on private land and inaccessible. 

The limey portions in the Cretaceous rocks sometimes contain beds and crystals of nice calcite.  The Smoky Hill Chalk Member of the Niobrara Formation often has joints filled with secondary crystals.  These seams extend for tens of feet and produce some really great calcite specimens.  Collectors might examine any Smoky Hill exposure, especially in northern Ellis County (off I-70, Exit 145), and chances are good for locating crystals.

Calcite crystals from seam in Smoky Hill Chalk, Ellis County.
Silicified beds in the Neogene Ogallala Formation/Group have been collected since the early part of the 20th century, mainly from Kansas, but also in parts of Nebraska and Texas. The best known of these silicified beds is the aptly named the “Green Quartzite”, a quartz- to opal-cemented sandstone and/or conglomerate that forms the local ”caprock” in many western Kansas localities.  Of greater interest to rockhounds are the concretions and beds described by Frye and Swineford (1946) as irregular masses (up to 8 inches in long diameter) of dense, cream-colored, waxy or resinous opal…containing vugs lined or filled with the more common translucent opal and some chalcedony, and on the outside consists of dull white porous silica… The rock is brittle and breaks easily with pronounced conchoidal fracture into small splinters. The current thought is that the source of silica was the vast beds of volcanic ash scattered throughout the Ogallala. Essentially this opal is a weathering product---silica leaching downward from the overlying ash beds.

The opalized nodules collected ~15 miles south of Wallace, Kansas, are among the most beautiful of the opalized concretions that I have observed. Some of the translucent opal is almost, or may be, gem quality. Although small, “moss” dendrites (manganese oxide) are also present.  I am unaware of jewelry, cabochons or faceted stones from Ogallala opal. But---there is always a first time!
The "moss opal" or "moss agate" from Gove County Kansas.  This slice is more of a dendritic chalcedony and is not very "opalized."  Width ~4.3 cm.

Opalized sandstone from Ogallala Formation, Wallace County.  Arrows point to "opalized" silica while the X indicates a lighter green silica with black dendrites.  These are the source of Kansas "moss agates".

Localities that mineral collectors commonly overlook are the Pleistocene sand and gravel deposits situated along the major river systems in the western part of the state, especially the Republican, Arkansas, Smoky Hill, and Saline Rivers. These streams have cut through and eroded the Ogallala Formation and then deposited their load (as current drops in velocity) in flood plains and channels.  Today these deposits are often well-exposed in numerous sand and gravels “pits”.   If these excavations are not available, simply walk the stream beds---See Blog Posting June 4, 2011. The "moss opal" or "moss agate" from Gove County Kansas.  This slice is more of a dendritic chalcedony and is not very "opalized."  Width ~4.3 cm.

I have found a variety of minerals in the gravels with the finest being various shades of jasper ranging from red to orange to green to brown.  Some of these jasper specimens represent silicified chalk from the Cretaceous Smoky Hill Chalk.  These original jasper outcrops are scattered across northwestern Kansas and seem to represent a post-depositional enrichment of the chalk, possibly with some dissolution of the chalk, by ground water rich in silica.  The source of the silica—probably volcanic ash or bentonite (altered ash). 
Petrified wood is fairly common in the sand and gravels with the replacing material seemingly jasper rather than chalcedony.  Fossil fragments from the Cretaceous rocks are numerous as the prismatic structure of Inoceramid shells (large Cretaceous clams) are quite durable, and occasionally a “shark’s tooth” turns up.  Of additional interest are the “Kansas Diamonds”, pieces of rounded crystalline, often clear, quartz.  I find it interesting that most quartz fragments are quite rounded while the jasper is very angular.  Feldspar fragments are common but are much smaller in size that the various quartz minerals.
I have seen beautiful faceted specimens of quartz derived from “Kansas Diamonds” and combined with silver settings in quite spectacular jewelry settings.  In addition, the jasper makes wonderful cabochons or just plain tumbled stones. Native Americans often used the silicified chalk for projectile points and one sees the terms Niobrarite or Smoky Hill Jasper used in the literature.

Silicified chalk (Smoky Hill Chalk) or jasper collected from Pleistocene gravels, Trego County.
And finally, the rockhound scouring Kansas always has the opportunity to find a meteorite, or at least a fragment.  Nearly 150 meteorites are known from Kansas (Washington University, 2011) including one located in 2005 that weighed in at 1400 pounds (part of the Breham fall).  Check out the website: www.kansasmeteorites.com/.

In summary, Kansas is a state that many non-local rockhounds overlook in their quest for interesting specimens.  However, there are numerous available sites that provide a variety of minerals.

Frye, J. and A. Swineford, 1946, Silicified Rock in the Ogallala Formation: Kansas Geological Survey Bulletin 64, pt. 2.

Washington University in St. Louis, 2011, Meteorites in the United States: http://meteorites.wustl.edu/numbers_by_state.htm