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.