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May-June 2014

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Letters

CALL FOR PAPERS

The thirty-sixth annual symposium held in conjunction with the Tucson Gem and Mineral Show will take place on Saturday, 14 February 2015. The symposium is cosponsored by the Tucson Gem and Mineral Society, the Friends of Mineralogy, and the Mineralogical Society of America. As a tie-in with the show, the symposium theme is the same as the show theme: Minerals of Western Europe. Presentations on descriptive mineralogy, classic and new localities, and related subjects are welcome. An audience of amateur and professional mineralogists and geologists is expected.

Anyone wanting to present a paper should submit a 200–300-word abstract to Julian C. Gray, Tellus Science Museum, PO Box 3663, Cartersville, GA 30120; email juliang@tellusmuseum.org; phone 770/606-5700, ext. 415. Presentations will be twenty minutes in length. Abstracts must be submitted by 31 August 2014.

I would also like to take this opportunity to thank those who made the 2014 symposium a success, including speakers Henry Barwood, Victor Garanin, Jim Hurlbut, Margi Jenks, Pete Modreski, Jeff Post, John Rakovan, and Penny Williamson.

LAS CHOYAS UPDATE

On 15 July 2013, I received an email from Jeannette Carrillo of Gem Center, USA, Inc. stating that it was “fairly safe to travel to the Las Choyas geode mine” in Chihuahua, Mexico. This was welcome news because my last trip to the mine was in November 2006, just one month before the official declaration of the Mexican Drug War. I made plans to return in November 2013.

Around 2008, Gem Center had suspended underground mining activities due to moderate earthquake activity centered in Chihuahua. At that time, they had been mining between 90 and 125 feet beneath the desert floor. In order to maintain production, Gem Center purchased excavation equipment to begin strip mining along a portion of the deposit outcrop. This decision has been successful and productive. To date, an area approximately 300 feet long has been surface mined along the outcrop of the deposit, and the current high wall measures about 20 feet. The excavation has also exposed the classic room-and-pillar mining technique that has been employed underground since the 1960s.

In the summer of 2013, because seismic activity had subsided, two new shafts were sunk downdip to probe the depths of the deposit. The geode-bearing zone was encountered at 150 feet and 210 feet below the surface. The two shafts have been connected with a tunnel, and they have been mining along strike. Potentially, the geode zone could be mined down to 600 feet below the desert floor, where the regional water table would be encountered. At the writing of this letter (late fall, 2013), mining of geodes is occurring at the surface and underground at the deposit.

I was lowered down the 150-foot shaft like a bucket into a well and was able to shoot video footage and still photos of mining activity—I even collected a geode that contained amethyst! A return trip is planned for fall of this year.

DIAMOND HARDNESS

This letter is written to further explain the hardness difference in diamond faces as mentioned in the article “Coming to Terms with Diamonds: A Matter of Facts” by Carl A. Francis and me (January/February 2014, pages 31–33). Determining diamond hardness has always been problematic. Standard indentation tests such as that of Knoop, Rockwell, Vickers, Shore, and Brinell failed because no indenter material had sufficient hardness to make an impression on a diamond crystal. The cubic face of a diamond is harder than all other known materials. But the octahedral face of diamond was known to be even harder. Because indentation tests were insufficient in determining the relative hardness of the diamond faces, researchers relied on comparing grinding resistance to determine relative hardness.

Grinding/polishing a diamond using diamond abrasives indicated large differences in hardness between the cubic and octahedral faces, with variations depending upon grinding direction/orientation (what diamond cutters call “cutting against the grain” or “with the grain”). Denning reported measurable grinding hardness differences on the octahedral faces in excess of 500 times harder than the softest orientation of the cubic face using grinding methodology. His upper limits of hardness were interpolated because the hardest face-orientations failed to show any satisfactory results during grinding (see figs. 3 and 4 in American Mineralogist, 1953, vol. 38, pages 108–117).

However, the discovery of ultrahard fullerite (C60) provided a material that was harder than the hardest face of a diamond. Fullerite has a hardness of 310 gigapascals (GPa) on the Vickers scale. Indentation tests using a fullerite indenter finally were able to yield hardness results for the diamond cubic face of 137 ± 6 GPa and on the octahedral face of 167 ± 5 GPa, a difference of approximately 20 percent (Journal of Materials Research, 1997, vol. 12, pages 3109–3114).

This small difference in relative hardness makes a large difference when polishing diamonds. Diamond abrasives easily polish cubic faces in the correct grinding direction but slowly polish octahedral faces even in the best grinding direction, and no polish is possible in certain grinding directions. Until ultrahard fullerite abrasive becomes available, the 20 percent hardness difference between the faces will still result in large differences in polishing time.

AFMS PROGRAM COMPETITION

The American Federation of Mineralogical Societies (AFMS) is pleased to announce the winners in the 2013 program competition. The winning programs are available to affiliated clubs from their regional program library. (Note: Final titles may be slightly different than those shown.)

Class 1—Educational Presentation, First Place with Honors: Agates Under the Microscope, DVD, approximately 30 minutes (with narration), by Doug Moore, Heart of Wisconsin Gem & Mineral Society (Midwest Federation).

Class 1—Educational Presentation, First Place: Turquoise of Nevada, DVD, approximately 30 minutes (with narration), by Terry Vasseur, South Bay Lapidary & Mineral Society (California Federation).

Class 4—Just for Juniors. First Place: Jade—Digging Deep Isn't Enough, DVD, approximately 5 minutes (with narration), by Hannah Zakharenkov, Roseville Rookie Rock Rollers (California Federation).

To participate in the AFMS program competition, see the rules and guidelines at www.amfed.org, or email me at margaret@qtm.net for information.


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