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Students examining large spodumene crystals in the Etta mine

Photo source : unknown

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Spodumene - Spodumen - Spodumene - Espodumena

North America : USA : Giant spodumene crystal "logs"
up to 14 m length in the Etta pegmatite mine

Coordinates : 43°52'N , 103°25'W : Etta mine, Keystone, Pennington Co., South Dakota, USA

 

The Etta mine near Keystone in the Black Hills began operations in 1883 as a mica mine. Accompanying "black ore" turned out to be rich tin ore and a rush began which resulted in the claiming of several other mines in the vicinity. However no substantial tin ore bodies were discovered and most mines were abandoned soon. In the Etta however the tin boom at least proved the existence of large quantities of the lithium ore spodumene, which was mined separately since 1898.

The Etta mine operated until 1959. It became famous for its giant spodumene crystals, which form individual "crystal logs" in a quartz - feldspar - mica matrix. These spodumene logs reached remarkable dimensions, whereas the first reference of the logs at the Etta mine was by Blake in 1883.Frank Hess in 1939, investigating rare minerals for the Bureau of Mines described these incredible crystals :

'huge crystals of spodumene are mixed at every possible angle like toothpicks in a translucent gel (quartz). In 1904, a crystal 42 feet long and 3 feet by 6 feet in cross section was found...The crystal weighed about 65 tons.'

Spodumene crystals of similar dimension were abundantly found in the course of lithium exploitation at Etta. But not only here : Giant Spodumene crystals of up to 10 m = 30 feet were discovered also in the nearby Ingersoll mine, accompanied by large orhoclase and beryl crystals as well as at the Tin Mountain Quarry

Today the closed Etta mine, with its gaping hole, is still a spectacular sight and large spodumene crystals can be spotted at the steep rock walls. However the mine is on private property and visitors are not welcome.

Giant white spodumene logs in
the walls of the Etta open pit

Photo : A.V. Morgan

Other notable & famous spodumene occurences :

- Another locality with giant spodumene crystals is the nearby Tin Mountain quarry, where a 10 m large spodumene crystal was discovered.

- A further well known north american spodumene locality is the Harding mine in New Mexico, where a multitude of up to 5 m long crisscrossing spodumene crystals in the pit walls are still acessible today. For more information click here.

- The largest known european spodumene deposit with rich reserves and many rare minerals is the well explored, but never seriously mined Weinebene pegmatite at the Koralpe, Carinthia, Austria

- Gemmy spodumene of the pink variety known as kunzite can reach up to 1 m crystal size and is reported both from Brasilia (photo as well as from the Nuristan region, Pakistan

Cross section of the Etta Mine in 1929 clearly showing the large
spodumene "logs" in the main pegmatite body

Rendered by G.M. Schwartz and reproduced from Economic Geology, vol. 20 (1925)

Todays view into the partly flooded Etta open pit

Photo source : Gary Arndt at www.flickr.com

Historic photo of a 14 m long spodumene "log" excavated in 1904

Photo source : G.M. Schwartz and reproduced from Economic Geology, vol. 20 (1925)

Historic photo of two spodumene "log" excavated around 1920

Photo source : United States Geological Survey

 

Etta mine - Essentials :


Mineralogy :
Spodumene, accessories are amblygonite, feldspar, quartz, mica, cassiterite

Crystal Size :
Spodumene crystal logs up to 14 m, and m - sized feldspar crystals

Geology & Origin :
Lithium and beryl rich pegmatites, crystallization from granitic melt

Current status :
Inactive mine on private property

Remarks :
A classic locality of giant crystals, now largely mined out. However, giant spodumene logs can still be observed in the walls of the open pit
Resources and relevant weblinks :

The Etta mine and many other pegmatite mines in the Black Hills have raised geological interest since a long time and today they constitute one of the classic geological field trip areas for many geological departments. Therefore there is a vast literature available about this area, which is probably one of the best documented 'giant crystal areas' worldwide. To name a few :

Blake, W. P. (1884) Transactions of the American Institute of Mining Engineers Vol. XIII: “Tin Ore Veins of the Black Hills”.

DeWitt, E., Redden, J. A., Burack Wilson, A., and Buscher, D. (1986) Mineral resource and geology of the Black Hills National Forest, South Dakota and Wyoming. United States Geological Survey Bull. 1580 

Guiteras, J. R. (1940) Mining of feldspar and associated minerals in the southern Black Hills of South Dakota. United States Department of Interior, Bureau of Mines, IC 7112. 

Hess, F. L. (1939) Lithium. United States Department of Interior, Bureau of Mines, IC 7054 

Landes, K. L. (1928) Sequence of Mineralization in the Keystone, South Dakota pegmatites. American Mineralogist v. 13, p. 519. 

Lincoln, F. C. (1927) Pegmatite mining in the Black Hills. Engineering and Mining Journal v. 123, n. 25. 

O’Harra, C. C. (1902) The mineral wealth of the Black Hills: South Dakota School of Mines Bull. 6 

Roberts, W. L., and G. Rapp Jr. (1965) Mineralogy of the Black Hills. South Dakota School of Mines bulletin 18.

Ziegler, V. (1914) The minerals of the Black Hills. South Dakota School of Mines bulletin 10.

Furthermore there is an excellent online publication on the subject :

Black Hills Pegmatites !!

by Thomas A. Loomis, published in : MATRIX Vol. 10, No. 3 Issue

which can be accessed here

 

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