GAHNITE: ZINC SPINEL.

Det har ingen betydelse hur sakta du går bara du inte stannar.

This is day 90-100 of pretty much staying at home playing with the minerals, reading, writing, and trying to stay free of Covid-19.  Some days are more boring than others but at least I stay well.  On most days, the minerals are the highlight as I dig in and try to continue my goal of remaining a lifelong learner. On a recent play day, I was looking at a specimen containing gahnite, a zinc aluminum oxide [ZnAl₂O₄] related to spinel.  My specimen came from California but the Type Locality for gahnite is the Falun Mine, Falun, Dalarna County, Sweden.  All of a sudden, the bells and whistles started going off in my ole brain. Why? 

The Kopparberget

Another item that holds my interest is family history and luckily, I have acquired a fairly substantial history on both sides of the family. One branch of the paternal side leads to Sweden. When migrating to the U.S. [ca. 1874-1884] my great grandfather settled in Falun, Saline County, Kansas, not far from my hometown.  In fact, there is a substantial Swedish community in that part of Kansas anchored by the city of Lindsborg.  Although my relatives that I remember growing up were farmers I wonder if any of the previous generations worked in the large Falun Mine?  That is one of life’s persistent questions and I will try and locate additional information.  Just a tad of serendipity here!

Lutheran Church, Falun, Kansas.  Photo courtesy Kansas Historical Society.

According to MinDat, the Falun Mine [AKA Kopparberget, Sweden’s Great Copper Mountain] in the 17^th century produced two-thirds of the world’s copper. Initial production started in the 11^th century and by the end in 1992 the mine had yield 500,000 tons of zinc, 400,000 tons of blister copper, 160,000 tons of lead, 380 tons of silver, and 5 tons of gold.  That is a lot of metal. The stratigraphy at Falun [part of the Fennoscandian Shield] is quite complex with original volcanic and sedimentary rocks of Precambrian age [1.8-1.9 Ga] intruded by granite, folded and faulted, and then invaded by hydrothermally emplaced sulfides (pyrite, chalcopyrite, sphalerite, and galena) when the hot fluid interacted with carbonates and mixed with cool seawater in a seafloor environment (Kampmann and others, 2017).

A fascinating history of the Falun Mine,  There Once Was a Goat Named Kåre—Over a Millenium of Mining at Falu Gruve, Sweden (Nathalie Brandes) may be found at https://zh.mindat.org/article.php/2912/There+Once+Was+a+Goat+Named+K%C3%A5re%E2%80%94Over+a+Millenium+of+Mining+at+Falu+Gruve%2C+Sweden

Gahnite is a member of the Spinel Group and is in a solid solution series with: 1) hercynite (ZnAl₂O₄ to FeAl₂O₄); and 2) spinel (ZnAl₂O₄ to MgAl₂O₄) and therefore usually displays octahedra crystals; however, it may also occur as massive or granular grains.  It is quite brittle and hard at ~8.0 (Mohs) and many grains/crystals appear fractured.  The color is usually dark green to dark blue (due to iron content) to black.  Most of the gahnite I have observed appears very dark or black and opaque (but translucent in thinner pieces) with a vitreous luster and a gray streak and the grains/crystals are small. Unlike spinal, gahnite is rarely faceted into gem cuts (mostly due to size of crystals).

Gahnite is an accessory mineral in granite or granitic pegmatites, metamorphosed sulfide ores, crystalline limestone subjected to contact metamorphism, and metamorphic schists. My specimen was collected from the Goodview Mine, a former gold-silver-copper working in the Green Mountain District of Mariposa County, California.  I could not locate much information about the mine except that it was a small producer in the early 1900s and access is now closed off due to water flooding.  MinDat noted the pyrrhotite-chalcopyrite ore was in a veins and fractures in a metamorphic sericite schist.

 Låt båda sidor försöka åberopa vetenskapens underverk istället för dess skräck. Låt oss tillsammans utforska stjärnorna, erövra öknarna, utrota sjukdomar, knacka på havsdjupet och uppmuntra konst och handel.  J.F. Kennedy

REFERENCES CITED

Kampmann, T., Jansson, N., Stephens, M., Majka, J., Lasskogen, J., 2017, Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen Ore District, Fennoscandian Shield, Sweden: Economic Geology v. 112, no. 5.

Arrows pointing to octahedrons of gahnite among many black grains/crystals as shown in photomicrographs below.  The largest crystal shown at the top arrow is ~ 1 mm.

Line drawing of a typical Spinel Group octahedron crystal.

Photomicrographs of gahnite crystals/grains.  Most are submillimeter in size.

RARE BARIUM SILICATES:SANBORNITE AND MACDONALDITE

Most rockhounds are familiar with the element barium although they have never seen the element in its natural state—it is never found in nature as a free element.  We know the element due to its combination with the sulfate ion, SO₄, to produce the mineral barite or baryte—BaSO₄, or to the lesser known barium carbonate, BaCO₃, the mineral witherite. Other than those two minerals many of us would be hard pressed to talk about other minerals containing barium as it is mostly an accessory element, or minerals with barium as the major cation are rare.

I certainly knew very little about barium minerals until I found an article by Dunning and others (2018) describing the distribution of barium silicate minerals from Baja California, Mexico, north to Alaska.  It is a comprehensive, three-part series and may be found at: www.baymin.org/papers.

Barium silicates are rare minerals and many of the described sites only contain one or two different species, yet these total sites contain 44 different minerals.  Even today there are new barium silicate minerals being discovered and described.  About the only minerals on the list that I recognize are joaquinite-Ce [NaBa₂Ce₂FeTi₂Si₈O₂₆(OH)-H₂O] and benitoite [BaTiS₃O₉] collected from the famous Dallas Gem Mine in the San Benito Mountains of California.  The blue gem benitoite is the State Gem of California.  Both minerals form in fracture fillings correlated with subduction zone rocks (serpentinite, greenstone, blueschist) associated with converging plate boundaries, mostly at high pressures and low temperatures.

After the Dallas Gem Mine the best-known locality for barium silicates is Big Creek-Rush Creek Mining District in Fresno, County. I could not locate much information about the mining history other than the mines produced barium.  To rockhounds, the interesting aspects of the District are the mines that have produced the type locality of 16 rare barium silicates plus they are the home of at least 13 other rare barium silicates.  By my count that is 29 different rare barium silicate minerals at this single locality. In examining mineral photos from Big Creek-Rush Creek on MinDat it is interesting to note that many of these barium silicates only have three to four photos displayed on the web site.  Although the number of photos on MinDat is not a solid indicator of mineral abundance, it certainly gives the reader a decent indicator of rarity to abundance. 

 

From a Denver Show many years ago I picked up a perky box with a specimen containing the barium silicates macdonaldite and sanbornite collected from the Big Creek-Rush Creek District in California. I purchased it since the District is the Type Locality of macdonaldite (the wonders of cell phones to examine MinDat when looking at minerals).  It certainly was worth the two bucks I paid, and there are probably additional barium silicates in the specimen.  

Macdonaldite is a barium calcium hydrated silicate [BaCa₄Si₁₆O36(OH)₂-10H₂O] that has a silky luster (may appear at times to be vitreous). It is soft at ~3,5-4.0 (Mohs), usually white to perhaps colorless, and is transparent to translucent.  MacDonaldite usually appears as acicular crystals or fibers arranged as a white radial group.  At Big Creek-Rush Creek macdonaldite appears as veins and fracture coatings in a sanbornite and quartz bearing metamorphic rock, the result of contact metamorphism of sedimentary rocks by a Late Cretaceous granodiorite pluton (Dunning and others, 2018).  Macdonaldite is a low temperature and late appearing mineral and may, in most cases, be an alteration product of sanbornite (Dunning and others, 2018).

 

A large radial group of macdonaldite crystals and fibers located on sanbornite.  Width FOV ~9 mm.

A small group of macdonaldite acicular crystals (M) less than 1mm in width. Numerous other minerals, perhaps barium silicates.

I presume the brown to brown-yellow mineral surrounding macdonaldite (M) (~1 mm) is a barium silicate (maybe even two or more). Perhaps it is verplanckite?

 

Could the orange be muirite?

Sanbornite, a barium silicate [Ba₂(Si₄O₁₀], is colorless to white or perhaps pale green, but forms platy sheets with good cleavage.  The sheets are often iridescent and have a vitreous to pearly sheen.  It is transparent to translucent with a hardness of 5.0 (Mohs). Sanbornite occurs in veins of metamorphic quartzites (previously sandstone) and hornfels (previously shale/siltstone; low pressure; moderate to low temperature ~400-600 C) (Dunning and others, 2018) heated by igneous plutons, and almost always occurs with quartz.

 

A stack of horizontal sanbornite sheets with the arrow parallel to the sheets. The white coating is probably macdonaldite. Width FOV ~1.4 cm.

Looking at the top layer of the stack shown above.  Note pearly to vitreous luster and the iridescence. Width FOV ~1.4 cm.

I have been trying to compare the barium silicates with the calc-silicates; however, that may be above my pay grade.  The calc-silicates [Ca₅(SiO₄)₂(CO₃)] usually form in high-temperature, contact metamorphic zones where a granitic-dioritic magma (high magnesium, silicon, aluminum, and iron content) intrudes into cooler (lower temperature) limestone or other carbonate rocks.  The invasive hydrothermal fluids alter the limestone into other minerals such as iron oxides, calc-silicates (wollastonite, diopside), andradite and grossularite garnets, epidote and perhaps ore minerals. These altered carbonate deposits are termed skarns. 

The barium silicates also form in low to high temperature, low pressure contact metamorphic zones.  My question involves the original source of the barium—where did it originate?  I did find out from Dunning and others (2018) that mixtures of BaCO₃ plus SiO₂ yielded BaSi₂O₅ plus CO₂ according to the equation: BaCO₃+ 2SiO₂→ BaSi₂O₅+ CO₂↑. This equation shows witherite and silica produced sanbornite plus CO₂ vapor at a temperature range from 440 C to 600 C.  Other stoichiometric mixtures of silica + baryte failed to react at temperatures up to 750 C. Virtually no baryte broke down at these temperatures. These results would appear to eliminate baryte as a direct source of the barium for the formation of sanbornite during metamorphism.  The reaction of barium carbonate and silica to form sanbornite and carbon dioxide is analogous to the well-known reaction of calcite and silica forming wollastonite and carbon dioxide. So, does the formation of all barium silicates require the presence of barium carbonate? 

If I understand Dunning and others (2018), the presence of barium carbonate was required for the formation of most barium silicates although a few minerals are the result of secondary alteration of sanbornite or gillespite. 
 

1.    Sedimentary baryte, or witherite, precipitated locally as part of an abyssal sedimentary sequence in a possibly continuous narrow basin off the west coast of Mexico and California within the late Paleozoic to Early Jurassic time period.

2.    Diagenetic activity then dissolved the baryte and re-precipitated the barium as witherite.

3.    Burial of the sediments to depths was followed by one or more periods of severe deformation, probably caused by subduction along a continental margin and thermal metamorphism attributable to the intrusion of large granitic plutons. The temperature of metamorphism was between 400 C and 600 C.

4.    Metamorphic action on residual sediments rich in barium (the witherite) and other elements is the major reason for the majority of barium silicate occurrences.

5.    The majority of barium silicates identified in this study are of primary origin.

6.    The secondary alteration of sanbornite during low temperature hydrothermal activity has produced macdonaldite.

In my case, this is  one of life’s persistent questions that has been ferreted out by locating the tremendous article by Dunning and other (2018).

REFERENCES CITED

G.E. Dunning, R.E. Walstrom, and W. Lechner, 2018, Barium Silicate Mineralogy of the Western Margin, North American Continent, Part 1: Geology, Origin, Paragenesis and Mineral Distribution from Baja California Norte, Mexico, Western Canada and Alaska, USA: Baymin Journal, Vol 19, No. 5.