TUCSON SHOWS AT LAST: TOURMALINE

 
Well, it has finally arrived, or at least I have arrived in Tucson to bum around at the Tucson Gem, Mineral and Fossil Shows.  At my best count there are ~60+ different venues/shows this year anchored by the Tucson Gem and Mineral Show® (TGMS) scheduled for the Tucson Convention Center February 8-11.  Most of the other venues/shows run during the two weeks preceding the end of the TGMS—in other words about the first two weeks of February, give or take a few days on either end.  My goal this year is to visit 15 or so of these ancillary shows.  Some of these other venues are huge; for example, the G&LW Tucson Show/Holidome has over 500 booths (as best as I count count) while other small shows have a half-dozen booths.  Some shows are in hotels and utilize individual rooms while others, such as the large 22^nd Street Show, erect giant, and I mean really large, tents.  Other more “mom and pop” shows have dealers selling from trailers or individual sun/rain protectors or pickup beds open to the elements.  There are several venues that specialize in specific areas---perhaps beads, fossils, high-end minerals; however, most are a mixture of “everything” from clothes to rugs to dinosaur mounts to all sort of minerals and rocks.  In addition, specific shows such as the large JOGS Tucson Gem and Mineral Show are restricted to wholesale buyers and “buyer tags” are required for entrance. All in all, the shows/venues represent an amazing experience.

One of the first shows I headed for was the 22^nd Street Mineral Fossil and Gem Show, a cornucopia of pleasures.  My major purchase was a grouping of Brazilian tourmaline crystals.  No, the crystals were not gemmy but I liked the piece for display and it was in my monetary range (cheap) of $5. 

One of the "big" tents at the 22nd Street Show.

I have always enjoyed the brecciated jasper termed mookite collected in Australia.

Almost all shows have Eocene fish collected from the Green River Formation near Kemmerer, Wyoming.

How about a sting ray from the Green River quarries.

There are always displays of magic minerals.  In a couple of weeks I am taking a piece of amethyst and heading to a vortex near Sedona.

You can always find a variety of merchandise at the 22nd Street Show.

Tourmaline is now classified as the Tourmaline Group with several end-member species created by common solid solution series.  Virtually every member of the Group seems in solid solution with other members.  Every few years a new classification pops up.  For example, on 1 February 2018, I noticed an abstract by Bosi stated that his new study “has direct implications for the tourmaline nomenclature, as well as on petrogenetic and provenance information. Some assumptions behind the classification scheme of tourmaline have been reformulated, revealing major agreement and significant improvements compared to earlier proposed scheme.”  So, if you want the newest scoop locate his paper in the American Mineralogist. Otherwise I will offer some generalities.

Tourmalines (refers to the Group) are boron silicate minerals with a large variety or combination of calcium, potassium, sodium, aluminum, iron, lithium, vanadium, magnesium, manganese, chromium cations (the positive charged elements) thrown in. For example, the well known variety elbaite is: Na(Al_1.5Li_1.5)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃OH. The chemical formula is really hard to understand but I will toss out the general formula from www.mindat.org:

                  A(D₃)G₆(T₆O₁₈)(BO₃)₃X₃Z

A = Ca, Na, K, or is vacant (large cations);
D = Al, Fe²⁺, Fe³⁺, Li, Mg²⁺, Mn²⁺ (intermediate to small cations - in valence balancing combinations when the A site is vacant);
G = Al, Cr³⁺, Fe³⁺, V³⁺ (small cations);
T = Si (and sometimes minor Al, B³⁺);
X = O and/or OH;
Z = F, O and/or OH.

 

As you can guess there is much cation substitution, so the group has many different varieties; MinDat lists 39 different “kinds” of tourmaline.  A couple of members that many of us recognize would include the common black schorl (iron rich form and the most common variety in rocks) and the pink elbaite. Bosi (2018) noted that “tourmalines form the most important boron rock-forming minerals on Earth.”

Pink elbaite crystals in a off white matrix that is some sort of a feldspar mineral (maybe microcline, maybe not). Width of photo ~8 cm.

Elbaite, a sodium lithium aluminum boron silicate (in the formula above sodium would occupy Position A of large cations while lithium and aluminum occupy Position D of smaller cations). It is probably the most colorful of any of the Group members and in some case even bi-or tri-colored (for example the popular watermelon tourmaline with zoned crystals displaying a green outer layer over a pink center).  The best known colored elbaite is probably the pink variety (sometimes termed rubellite), and that is what I purchased. Elbaite belongs to the Hexagonal Mineral System and often is found as elongated prismatic, striated crystals without cleavage.  They also have a very distinguishing rounded triangular cross section.  Gemmy varieties are vitreous and transparent while less gemmy crystals are more translucent.  Tourmaline is about the hardness of quartz at 7.0-7.5 (Mohs) and gemmy crystals of many colors are in high demand for faceting, carving and cabochons. 

The seller stated that the specimen was from one of the mines in the State of Minas Gerais, Brazil-----an area of hundreds or thousands of mines.  MinDat states that “probably the best known of these are the pegmatite minerals like tourmaline, beryl, quartz etc. that come from the pegmatites in the north west granitic region of the state and whose center, business wise if not geologically is the town of Governador Valadares.”  So, I will settle for that general area!

The pink color of elbaite is probably due to the high lithium content; however, this variety also occurs in about every color of the rainbow and I am uncertain about other color chromophores.   In fact, I have heard of elbaite referred to as lithium tourmaline.  I also understand that the lithium (and the boron) is very important in lowering the magma solidification temperature.  

  

So, the Tourmaline Group is a very complex assemblage of minerals and understanding much of the mineral chemistry is above my pay grade.  However, the various tourmalines often make spectacular gemstones and display specimens.  I was impressed in observing a very large “pile” of these masses of crystals at the venue.

REFERENCES CITED

Bosi, F.,2018, Tourmaline crystal chemistry: American Mineralogist, v. 103.

RUBY-IN-ZOISITE: ON THE WAY TO TUCSON

Sunset in camp.

I noted in the last posting that we were in the holiday time of the year and presented my Christmas “wish list.” We had zoomed past Halloween and the pumpkins and broom-riding characters. The orange pumpkins represent the bounty of the fall harvest and the orange fires lighting up the darkening night sky.  The Celts and Druids visited their spirit world ancestors around Halloween (All Hallows Eve) while many Christian religions celebrate All Souls Day and All Saints Day at about the same time.  Spirit worlds are dark and black as are “witches”, black cats, black bats and black spiders.  My favorite orange of Halloween is associated with candy corn!   

Before the masks were off the discount shelves we slipped into a purely American holiday, Thanksgiving. Here we celebrated the bounty of the harvest, especially yellow corn, perhaps the most staple grain during all episodes of U.S. history.  The orange pumpkins were still around although various “punkin chunken” contests certainly helped delete the supply.  Brown is often associated with the holiday since most fall and late winter leaves are sort of this drab color.

On or about December 22nd or 21st the Winter Solstice arrived in the Northern Hemisphere: the sun reached its most southern position directly above the Tropic of Capricorn, (~23.5 degree south), and the days were short and dark.

A few days after the Solstice, the traditional Christian-celebrated Christmas arrived on December 25^th.  The colors most associated with Christmas seem to be red and green (with white a distant 3^rd) although in the modern world blue, silver and gold are thrown into the mix.  Since Christmas in the northern hemisphere arrives during the often bleak, drab and gray winter, early European civilizations brought sprigs of greenery, for example pine boughs, into the house as a way to cheer up the inhabitants.  Even better would be a find of holly with red berries.  As a substitute for holly, early Christmas trees were decorated with red apples.  White represents the external world of snow.

Now, there also are a number of Christian religious reasons for the red, green and white used at Christmas; however, I will not delve into those issues.

Things just sort of chugged along with Christmas colors and an elf-like, dour Santa until 1930 when an illustrator by the name of Fred Mizen was hired by the Coca-Cola ® Company to create a happy Santa drinking a bottle of coke ®.  That first happy Santa was used as advertising in the December 1931 edition of the Saturday Evening Post.  The image was a big hit with the public so Coca Cola hired, in 1931, an illustrator by the name of Haddon Sundblom to create new Santa and Coke images on a yearly basis,  That relationship lasted for 35 years as Haddon created a jolly, plump and happy, older Santa Claus.  Haddon dressed his Santas in red and white to match the advertising colors of Coca Cola, his employer.  Red and White were also the colors of robes worn by Christian bishops.  Haddon evidently also borrowed from Clement Moore’s 1822 poem, A Visit from Saint Nicholas—you know, Twas the night before Christmas and all through the house… chubby and plump, a right jolly old elf.

So today, a non-religious Christmas celebration is usually represented by a green tree (pine, spruce, fir) and a red and white Santa.  Sundblom’s Santas radiated warmth from a grandfather like person, one who loved children and so snacks were left behind by the nice kids.  Almost any person of my age living in North America would recognize Haddon Sundblom’s rendition of Christmas Santas.  Coca Colas advertising was a stroke of genius and as a boy I always looked forward to seeing the newest editions and would ask my father to please bring home a coke along with an advertising poster.

So, what does all this banter mean when it comes to minerals?  Probably nothing but as the days got shorter and darker I sometimes lounged around thinking how to relate minerals to some sort of a story.  After 328 postings I sometimes lose my concentration, but not on this one---what would make good Christmas minerals, related red and green combinations?  So, I started putting this little offering together but was waylaid by a number or other opportunities---like spending time with my family in a condo at the Colorado ski slopes.  Now, I don’t downhill (love x-country) but sitting by a fireplace with a good book is a great way to spend an afternoon/week and so it was, and the beginning of 2019 arrived and disappeared.  By then it was time to begin packing my 5^TH Wheel for a visit to the Tucson Gem and Mineral Show making certain to arrive in mid-January for a camp near the Superstition Mountains in Lost Dutchman State Park.  So, here I am still thinking I need to get this post out!

In my way of thinking, the perfect Christmas “mineral” would not be a single mineral but a semi-precious gem called ruby zoisite, ruby-in-zoisite, or anyolite.  Specimens are composed mostly of the green mineral zoisite (chrome variety) accompanied by opaque dark red ruby crystals and often by black crystals of the amphibole pargasite (formerly identified as the hornblende amphibole tschermakite).  Anyolite specimens are commonly used to make cabochons, beads, tumbled stones and sculpted objects (elephants seem to be a favorite). But any way you look at it, the red ruby presents a great contrast to the green zoisite—the perfect Christmas stone!

Zoisite is a calcium aluminum silicate [Ca₂Al₃(Si₂O₇)(SiO₄)O(OH)] that would probably be unfamiliar to most people except for a single gemmy variety.  Most zoisite seems nondescript and occurs in a wide variety of shapes from massive to prismatic crystals.  The elongated crystals that belong to the Orthorhombic Crystal System are perhaps the most common and they are striated along the C-axis (the long axis).  Zoisite crystals also commonly radiate out from a center with the entire mass forming a “circle” (two-dimensional view).  Crystals occur in a variety of colors from red and pink to green and blue to brown or gray---take your choice.  In anyolite the tiny zoisite crystal have a green tint imparted by trace amounts of chromium. Zoisite is fairly soft (as a gemstone) and is usually stated as 6.0-6.5 (Mohs).  It is brittle, transparent to translucent to opaque, and can have a vitreous to pearly luster. It is found in a wide variety of environments ranging from contact metamorphic rocks to regional metamorphic schists to granite pegmatites.  It seems to be one of those minerals that is tough to pin down.

This beautiful emerald-cut tanzanite ring is offered for sale by Diamondere Inc. The total caret weight is 4.97 and the price is $8277.  Photo courtesy of www.diamondere.com.

As I stated above, who would recognize zoisite except for the sought after gemmy variety tanzanite!  Discovered in the late 1960s in Tanzania, tanzanite can be a beautiful blue color and is a giant hit on the gem market ranking only below sapphire as the most popular blue gemstone. The blue color in tanzanite is produced by trace amounts of vanadium; however, the natural deep blue gems are scarce and today most gems are produced by heating brown or green tanzanite to a temperature of ~600 degrees C when the oxidation state of the vanadium changes and the blue color is emphasized.  Tanzanite gems are graded similar to diamonds—clarity, color, size, and cut.  The cleaner (no inclusions) and bluer the stone the higher the value.  Good cuts require numerous facets to bring out the brilliance.  These good stones bring prices of thousands of dollars per caret.  For a great series of articles on the grading of tanzanite gems check out the series of articles by Lapigems at       http://www.lapigems.com/Articles/Tanzanite-Color-How-it-Affects-Value.aspx.

Tanzanite is one of the most popular gemstones on the market but is rare as all stones are mined from about 8 square miles in an area around Mt. Kilimanjaro in Tanzania. It might have remained a minor gem after discovery if Tiffany and Company had not put on an advertising blitz to get the public interested in this great new gem!  The blitz worked and today the company TanzaniteOne mining Ltd. Is the major player and seems to have a huge influence of the price (as does the government of Tanzania, and natural disasters such as floods).

So, if zoisite, other than tanzanite, is relatively unknown then ruby, the other main constituent in anyolite, is one of the best know gems.  In fact, ruby may be the most popular, and the most expensive, of the colored gemstones (others argue for gem emerald, a green beryl). A clear, large, pigeon-blood red, and well-cut ruby can bring thousands of dollars on the gem market.  Unfortunately, the ruby in anyolite is fractured and nearly opaque and therefore of non-gem quality.  However, it still contrasts nicely with the green zoisite and is very showy in cabochons.

Ruby, an aluminum oxide, is actually a variety of the mineral corundum (Al₂O₃) where minor amounts of chromium provide the red color.  A second variety of corundum is the multi-colored gem, sapphire, colored by minor amounts of titanium and iron. The most valuable sapphires are corn flower blue in color; however, several other colors of sapphires appear on the market.  Any colored corundum that is not red is a sapphire.

Corundum is a name that is well known to beginning geology students as it occupies #9 on the Mohs Scale of Hardness—sandwiched between #10 diamond (the hardest) and #8 topaz.  Corundum can be found in both metamorphic and igneous rocks that are silica-poor but rich in aluminum. However, the mineral is so hard that many of the gemstones are panned from alluvial gravels as the remains of eroded parent rocks.  Crystals are often six-sided, tapering, hexagonal dipyramids.  Both gem sapphires and gem rubies generally come from southeast Asia.

The third component of anyolite is the amphibole pargasite, a sodium calcium aluminum magnesium silicate.  Actually, pargasite is a group name with several modifiers to indicate a dominant element.  I presume that the anyolite black (actually very dark green) pargasite indicates the presence of iron.  In a previous post (2/16/15), the pargasite I described from Pakistan was a a nice green color, perhaps with minor amounts of chromium?

I purchased my specimen of anyolite a couple of years ago at the fall Denver Show.  It was collected from the major anyolite locality in Tanzania, the Mundarara Mine.  MinDat also reports a minor collecting locality near Drosendorf-Zissersdorf, Austria.  As best I can determine, anyolite is the product of metamorphic activity.

Anyolite or ruby-in-zoisite.  The zoisite, (Z) is composed of tiny prismatic crystals.  Pargasite (P) crystals are larger (up to 3 mm) while the ruby (R) is badly fractured.  Width of photo ~10 cm.

So, there it is—the perfect Christmas rock.  But now it is 77 degrees here in the desert, the sun is shining, a jolly old Santa seems ages ago and the length of daylight has increased 21 minutes since the Solstice.     The Tucson shows are just two weeks away –happy days are here again!

ALL I WANT FOR CHRISTMAS IS:

I believe in Santa Claus. I'll tell you why I do

"cause I believe that dreams and plans and wishes can come true.

I believe in miracles, I believe in magic too.

As recorded by Dolly Parton

Well it is that time of year again when Santa decides if I have been naughty or nice.  I am hoping for a nice review although some of the recipients of my letters expressing concerns for their legislative and congressional votes may cast a naughty vote! On the other hand, I have shared some of my fossil and mineral specimens with interested children and have guided them with collections.  That may swing the nice vote over to my side.  We will see who Santa believes are most important—politicians or children!

Lithographic, LTD up in Denver (www.lithographie.org) has a plethora of great books that would look great in my collection:

·       Minerals and Precious Stones of Brazil ~$100

·       Collecting Arizona ~$80

·       Minerals of Mexico ~$40

A pegmatite field trip with David London.

A better understanding of mineral crystal systems.  

A subscription to Mineralogical Record ~$72.

Another 90 good years of mineral collecting for Bob Jones.

Changes in the BLM/USFS collecting rules for fossils.

Better regulations for clubs and individuals to establish mining claims on federal and state properties.

The return of Bears Ears and Grand Staircase-Escalante to national monument status.

Someone to name a new mineral mikenelsonite (this is a really wild one).

A better camera with the ability to do focus stacking photographs, and someone giving me great instructions.

A personalized field trip through Colorado with Pete Modreski.

Wire silver specimens collected from Kongsberg, Norway, and Frieberg, Germany.

Being able to visually identify different minerals classified as pyroxenes and amphiboles.

A little peace and respect in this troubled world.

A personalized short course in identification of phosphate minerals taught by Tom Loomis. I would then hope to identify the dimorphs phosphosiderite and strengite.

A special place in Hades for lying politicians and claim jumpers and….

A successful 2018 rock and mineral show for the Colorado Springs Mineralogical Society (and for all other clubs and organizations).

A chance to climb just one more Colorado 14er. But really just better walking legs.

A better digital microscope with camera.

But in thinking about the wish list, I was reminded of an author’s (unknown) quote:  Its not the presents that make Christmas so special, it’s the presence of those you love, whether they are with you in person or spirit.  In that respect I guess Santa believes ole Mike has been mighty nice!

Merry Christmas, Happy New Year or Happy and Merry whatever.  Have a great year collecting your favorite minerals.

BLACK HILLS: MORE FAIBURNS, TEEPEE CANYON AND GOLD

I am living out my adolescent dream of travel and adventure.
Tim Cahill

Hunting Fairburns, summer 1966.

I recently had the opportunity to hit the road on my annual two-week fall camping trip to the Black Hills of South Dakota.  Most readers certainly realize that I “fell in love with” the Hills during my academic stay at the University of South Dakota from 1965-67.  My Blog postings feature South Dakota more than any other state as I continue to explore the fascinating geology and physiography—along with brew pubs, coffee shops and non-chain eating establishments.  My major discovery this year was a small coffee shop located in a used book store in Douglas, Wyoming.  Wow, what a treat to find such a locality “on the road.”

An earlier summer trip to the northern Hills is documented in postings on August 9, 2017.  This fall trip took us back to the southern Hills with headquarters in Custer State Park at Legion Lake Campground.  This location gives me a chance to: visit collecting sites east of the Hills to explore for Fairburn Agates; pound on the limestone at Teepee Canyon collecting the parental site of the Fairburns; explore garnet localities west of Custer; pan for a little gold near George Custer’s camp sites on French Creek; visit the great rock and mineral stores in Custer, Hill City and Keystone; and relive my past geology adventures and travel (see Post October 15, 2017).  The latter are often enhanced by an aging brain!

Study what you love, and you’ll never have to work a day in your life.  It’ll be one great adventure.
David Gerrold

Pounding on the rocks at Teepee Canyon.

I can’t say that the limestone at Teepee Canyon produced any magnificent specimens this year; however, the joy of being alone and pounding away was certainly worth the trip.  For new Blog readers, the quarries at Teepee Canyon, and other nearby localities, produce chert nodules from the late Paleozoic Minnelusa Formation; some of the chert is “agatized” with holly leaf fortification agates.   The Minnelusa is described as “Light-brown to red and gray sandstone, solution breccia, limestone and shale [that is] Lower Permian and Pennsylvanian {in age] (DeWitt and others, 1989). In past years these agates from the Minnelusa were called “limestone agates” and most collectors were adamantly opposed to any relationship between limestone agates in the Hills and the Fairburn agates from the surrounding plains.  However, Roger Clark has constructed a detailed study of the Minnelusa--Fairburn relationship and any collector interested in Fairburn agates should read his book (Clark, 2009)---and the agate photographs are spectacular.

Teepee Canyon.  Width photo ~6 cm.

Teepee Canyon.  Width photo ~4.4 cm.

Teepee Canyon.  Width photo ~3 cm.

Teepee Canyon. Width photo ~3 cm.

Teepee Canyon. Width photo ~2.7 cm.

Another agate that crossed my knowledge path somewhere in the past was called the State Park Agate  (sometimes Game Lodge Agates) and referred to agates collected in Custer State Park near the Game Lodge.  Clark (2009) refers to these specimens as limestone agates since their source was also the Minnelusa Formation.  Since rock and mineral collecting is not allowed in the Park, any of these agates on display or for sale evidently were collected years in the past.  This year I happened to spot a tray of Park Agates for sale in a Custer rock and mineral shop—Ken’s Minerals.  According to the clerk, the original owner of the shop (opened 1936), Kenneth Spring, Sr., picked up these Park Agates decades ago when collecting was legal.  So, I purchased a single specimen for my collection.

State Park agate.  Width photo ~ 9 mm.

I previously reported (October 7, 2017) on the Pringle Agates, also limestone agates and probably from the Minnelusa.  According to local shopkeepers the Pringle Agates always appear to be “bleached.”  I don’t know if that is true since I have seen very few agates attributed to the Pringle area.  However, my single specimen is certainly “washed out.”

A "limestone agate" locally named Pringle Agates likely eroded from the Minnelusa Formation near Pringle, south of Custer. Width ~2.7 cm.

So, now it was out to the plains east of the town of Fairburn for collecting at the original Fairburn beds.  I experienced “luck” at these beds in past years going all the way back to summer 1966.  This year I picked up a pretty poor one (a few faint lines on the jasper), a second specimen that is small but with nice lines, and finally a third that a friend trimmed, stuck in a tumbler and turned out a decent agate.  But perhaps the most exciting find was a silicified fossil (where silica had replaced the original calcite hard parts) known as Chaetetes milleparacedus.   The chaetetids were long thought to be a type of tabulate coral where their long, slender tubes contained tabulae or cross partitions (Phylum Cnidaria).  However, studies in the 1980s determined the chaetetids were a type of sponge (Phylum Porifera).  Chaetetids are very diagnostic animals, easy to identify and became extinct at the end of the middle Pennsylvanian (~307 Ma.)

Fairburn Agate collected in the rough and then subjected to trim, grinding, polishing and a tumbler. Width ~3.3 cm.

Fairburn Agate collected "in the rough." Width ~ 8 mm.

Side (top) and cross sectional view (bottom) of a stylized specimen of Chaetetes. Courtesy of University of South Florida, http://etc.usf.edu/clipart/

Silicified and well-rounded specimen of Chaetetes milleparacedus collected in the Fairburn Agate beds.  Width ~ 4.5 cm.

The Minnelusa contains a diagnostic invertebrate fauna and silicified fossil shells sometimes appear with the Fairburn Agates out on the plains (Clark, 2009).  In fact, in 2014 I collected a fossil brachiopod and reported on the creature in a posting on October 7, 2014.  Whatever the case, Fairburns or no Fairburns, the beds are a place for relaxation just trekking around and noting the wild looking scenery.

Wandering I do.  A mile or so south of Custer, along the highway to Pringle (US 385), I was poking around in the roadside ditch looking for tourmaline (schorl) when I picked up a small piece of granite/pegmatite holding some larger garnets.  The dodecahedrons seem likely to be spessartines (manganese aluminum silicate) since they are black or dark brown in color and appear not to have a hint of red.  Of course, my mineralogy might be a little suspect and perhaps they are the iron aluminum silicate, almandine.

Garnets, spessartine?, collected south of Custer.  Largest individual has a width of ~1.2 cm.

A couple of years ago a collector from Custer kindly gave me a quantity of rock from the Bull Moose Mine (~ 3 miles southeast of Custer city) with instructions to break it open and try to find small phosphate minerals.  Periodically I drag out the matrix and pound away.  This fall, on a slow night in October, I cracked open a bundle of pyrite crystals and discovered some beautiful lilac to and clear crystals of a phosphate.  Now, the problem arises—were the crystals phosphosiderite (AKA metastrengite), FePO₄-2H₂O, or strengite, FePO₄-2H₂O?  Yes, that’s correct---both minerals have the same chemical formula, a hydrated iron phosphate.  Phosphosiderite (Monoclinic) and strengite (Orthorhombic) are dimorphous minerals, that is they have the same chemical composition but belong to different crystal systems. And at the Bull Moose Mine, the two are difficult to distinguish; however, Tom Loomis at Dakota Matrix Minerals (www.dakotamatrix.com), the Black Hills guru of phosphate minerals, noted that phosphosiderite shows striations parallel to the long C crystallographic axis.  Since I am able to identify the striations, I am calling these neat little crystals phosphosiderite.

Photomicrograph of lilac-colored elongated crystals of phosphosiderite collected from the Bull Moose Mine. Width of specimen at bottom of photo is ~ 1.1 cm

Phosphosiderite is exposed on a flat surface on right side of photo and indicated by a P and arrow.  Most of the specimen is pyrite, Py, with minor barbosalite, B. Total width of specimen ~3.3 cm.

Phosphosiderite is a secondary mineral produced when primary minerals containing phosphate oxidize and the ferrous iron (Fe⁺⁺) moves to ferric iron (Fe⁺⁺⁺).  Notice above that the phosphate ion (PO₄^{- - -)} in phosphosiderite has in minus 3 charge and balances the Fe⁺⁺⁺   Tom Loomis. over at Dakota Matrix. noted “triphylite is the primary phase [the primary phosphate mineral] which is very susceptible to hydrothermal oxidation and is the host for most of the secondary phosphates [such as phosphosiderite] from the Tip Top and many other pegmatites [such as the Bull Moose].  Triphylite, Li⁺Fe⁺⁺PO₄^{- - -}, contains ferrous iron with a plus two charge, and lithium with a plus 1charge as they balance the minus 3 charge of the phosphate. Upon oxidation the ferrous iron moves to ferric iron as the lithium leaches away.   

I also have a second specimen from the Bull Moose purchased from a rock and mineral shop in Hill City and labeled phosphosiderite.  However, I suspect it is strengite and another iron phosphate, barbosalite, Fe⁺⁺Fe⁺⁺⁺₂(PO₄)₂(OH)₂ .

Photomicrograph (above) with stringers and"balls" of strengite; however, there are two color patterns present and it may be a mixture of the dimorphs strengite and phosphosiderite.  The bottom photos indicates the position of the strengite in massive barbosalite.  The ? is an unknown mineral, at least to me.

Pyrrhotite is a common mineral in the Homestake Mine of the northern Black Hills and an iron sulfide (Fe₇S₈ as listed in MinDat.org); however, it seems to have a variable amount of iron present (Klein, 2002) and is often listed as Fe_1-XS. Pure iron sulfide (FeS where x in the formula is 0---50 atomic percent iron) is a mineral called troilite that is quite rare on Earth but seems common in meteorites. There seems to be a complete solid solution between troilite and “typical” pyrrhotite where x in the formula is .2---44.9 atomic percent iron (Klein, 2002).  That indicates the pyrrhotite has about 20% vacancy of iron in the crystal structure.  For reasons that seem unclear to my mineralogical mind, troilite is non-magnetic while pyrrhotite is magnetic.  Perhaps not as much as magnetite but never-the-less, magnetic, and easily attracted to a magnet.  In the solid solution series, the magnetism then varies with the iron deficiency (more iron, less magnetism) but if heated to ~320^OC all magnetism disappears.

Pyrrhotite and quartz from the Homestake Mine. Total width ~3 cm.

Nice crystals of pyrrhotite are brassy to a dark brown in color.  They have a metallic luster and an almost black streak on unglazed porcelain.  Hardness seems to range on either side of 4 (Mohs).  However, this specimen I picked up at a rock shop in Hill City contains massive pyrrhotite and is labeled: Pyrhotite (sic) collected on the Rocky Mtn GSA field trip 4/3/60 3650” level, Homestake mine Lead, South Dakota.  Unfortunately, the collector is not identified. Interestingly this specimen was collected nearly 60 years ago!

Roberts and Rapp (1965) stated “pyrrhotite is one of the most abundant and most characteristic ore minerals of the Homestake Mine.  It is never found in crystals, but occurs in irregular, vermicular masses…”

I also picked up a mounted specimen (Perky Box) of galena and calcite cubes labeled Homestake Mine 1961. I nabbed on to this due: 1) to the older date; and 2) to my memory believing galena was somewhat scarce at Homestake.  Galena is listed on MinDat.org as being present; however, photos are not shown.  Roberts and Rapp (1963) noted only the presence of small cubic crystals, <1mm, in a vug.

Galena and calcite from Homestake Mine.  Total width of specimen ~2.4 cm.

Another specimen of cubic galena was purchased and labeled as collected from the Double Rainbow Mine near the settlement of Galena in the northern Black Hills.  I love the connection between the mineral and the settlement although the pay zone was silver.  As with many gold/silver/lead prospects in the northern Black Hills, the mineralizing fluids are associated with the Tertiary intrusives---with metal formation in the bedding planes, cracks, crevices and vugs of the Cambrian Deadwood Formation.  The intrusives at Galena are composed of numerous sills and dikes that contain a wide variety of rocks with a fine-grained groundmass sprinkled with feldspar phenocrysts.  Check out the Blog Posting on October 22, 2016.

Galena and sphalerite from Double Rainbow Mine. Total width ~3.0 cm.

Finally, in a previous posting (October 7, 2016) I detained some accounts of George Custer’s trip to the Black Hills in 1874 and his “discovery” of gold on French Creek near the town of Custer.  In that post I noted that a flake or two of gold was still available to “wannabe gold panners.”  Well, the take was a little better this year!  I could never “get rich” off panning gold but do understand how early miners might get “gold fever” when first observing those tiny flakes.

Several small nuggets panned from Custer County.  Width of photo is ~1.0 cm.

REFERENCES CITED

Clark, R., 2009, South Dakota’s state gemstone—Fairburn Agate: Silverwind Agates, Appleton, Wisconsin.

DeWitt, E., J.A. Redden, D. Buscher, and A.B. Wilson, 1989, Geologic Map of the Black Hills area, South Dakota and Wyoming: U.S. Geological Survey Miscellaneous Investigation Series Map I-1910. 

Klein, C., 2002, The 22nd edition of the Manual of Mineral Science (after James D. Dana): John Wiley and Sons, Inc.

Lufkin, J.L., J.A. Redden, A. Lisenbee and T. Loomis, 2009, Guidebook to the geology of the Black Hills, South Dakota: Golden Publishers, Golden, Colorado.

Roberts, W.L. and Rapp, G. Jr., 1965, Mineralogy of the Black Hills; South Dakota School of Mines and Technology, Bull. 18.