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November-December 2017

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Letters

DRAVITE and MERELANIITE

On page 382 of the July/August 2017 issue of Rocks & Minerals a picture of the new mineral merelaniite is presented (Jaszczak 2017). The merelaniite crystal is nested in a pocket of pale mint-green crystals that were speculated to be tremolite. Since that was published, several fragments of the associated mineral were analyzed and found to be dravite. Above is a close-up image of one of these crystals (fig. 1). Qualitative chemical analysis was done by EDS on an SEM (fig. 2) and shows the presence of Si, O, Mg, Al, Ca, Na, and V (the presumed chromophore for the green color). A single crystal X-ray diffraction dataset was collected and the structure was solved and refined to R1 = 1.69% with a hexagonal unit cell of a = 15.909(1), c = 7.1914(5) in space group R3m. The chemical formula based on site occupancy refinement, using all elements detected by EDS with the exception of V is (Na0.76,Ca0.24)(Mg0.60,Al0.40)3(Al0.78,Mg0.22)6(Si6O18)(BO3)3(OH)3O (hydrogens were not modeled). This gives a composition that is dravite dominant with a substantial uvite component.

Dravite, merelaniite, and graphite: note the trigonal morphology of the green dravite crystal. The field of view is 0.43 mm. John Jaszczak specimen and photo.

Figure 1. Dravite, merelaniite, and graphite: note the trigonal morphology of the green dravite crystal. The field of view is 0.43 mm. John Jaszczak specimen and photo.

Energy dispersive spectrometry (EDS) spectra from dravite. Analysis was done using a Hitachi S4700 field emission scanning electron microscope operating at a 30 kV accelerating potential and 10 μA beam current. The carbon (C) peak is likely attributable to the sample mounting tape.

Figure 2. Energy dispersive spectrometry (EDS) spectra from dravite. Analysis was done using a Hitachi S4700 field emission scanning electron microscope operating at a 30 kV accelerating potential and 10 μA beam current. The carbon (C) peak is likely attributable to the sample mounting tape.

Overall, the specimen is approximately 8 × 6 × 4 cm, with a matrix of milky quartz, transparent calcite, prehnite, and graphite covered by clusters of yellow prehnite crystals. Later-stage minerals include white laumontite laths, blocky white chabazite crystals, prismatic green dravite crystals, and silvery merelaniite whiskers. Minor associations include pearly white mica flakes, white acicular tremolite fibers, and a few anhedral grains of zoisite (tanzanite). The specimen is rather unique in several respects. First, it is relatively rich in merelaniite whiskers, which are confined to the dravite/chabazite area of the specimen. Although dravite has been reported from the Merelani gem mines, this specimen represents the first known to us that associates dravite with a suite of other species that confidently affirm its origin to the tanzanite gem mines as opposed to other known dravite localities in the region.

CALIFORNIA GOLD

I read with interest the article by Wayne Leicht in the July/August issue of Rocks & Minerals. In it he considers that the fine gold specimen under discussion formed in part in an open space and was never encased by vein quartz. Many years ago I consulted for a company that had the Georgia Slide area (where this specimen was found) under lease. Many core holes were drilled, and visible gold was very rare. However, a few did have coarse gold, and there was colorless to pink carbonate associated with the quartz veining. I suspect that this gold specimen formed within a quartz-carbonate vein and that the carbonate portion simply weathered away as the specimen-bearing segment of the vein neared the surface. I have seen similar carbonate associated with gold at the Eagles Nest mine where carbonate alteration of the host rocks is locally intense. Finally, I refer the readers to a Rocks & Minerals article written by John White (January/February 2004, pages 62–63) concerning gold pockets in which he makes it clear that this sort of crystalline gold likely never forms in an open pocket.

CORRECTION

Page 140 in Volume 83, Issue 2 (March/April 2008) incorrectly lists Robert Cook as the author of “Connoisseur's Choice: Olivenite, Tsumeb Mine, Tsumeb, Oshikoto Region, Namibia.” The correct author for this article is:

ROB BOWELL

SRK Consulting (UK) Ltd.

Churchill Way, Churchill House

Cardiff CF10 2HH, Wales

United Kingdom

rbowell@srk.co.uk

REFERENCE

Jaszczak, J. A. 2017. The where of mineral names: Merelaniite, Merelani (Mererani), Manyara region, Tanzania. Rocks & Minerals 92: 382–85. 

ORCID

John Jaszczak  http://orcid.org/0000-0002-1946-2454

 

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