Paper No. 7
Presentation Time: 8:00 AM-12:00 PM
WHY IS SOME CERUSSITE YELLOW, AND WHY DO WE KEEP BLAMING CHROMIUM?
The informal designation chrome cerussite has been assigned to yellow and green varieties of the lead carbonate mineral cerussite. This convention is not a formal species, but has become so entrenched that many museums use this descriptor in displays of yellow cerussite. The first documentation of chrome cerussite at the Magnet Mine, Tasmania, attributed the color to the presence of [Cr VI] as the blowpipe gives reactions for chromic acid (Petterd, 1903). Petterd also noted that green cerussite from the Adelaide Mine, Tasmania, contained only traces of Cr. It was later noted that yellow cerussite samples are invariably contaminated with the lead chromate mineral crocoite, which imparts a canary-yellow color (Anderson, 1907). Yellow to green cerussite is common in the mines of the west coast of Tasmania, Australia. In addition the yellow variety of cerussite also occurs at Mibladen, Morocco, Tiger, Arizona, U.S.A., and China. Chromium is not likely to substitute for lead in cerussite as it violates several of Goldschmidts Rules of Substitution. First, the size of the substituting ions differs by over 35%. Secondly, the normal state of chromium in the oxidation zone is Cr+6, leaving a charge imbalance that precludes direct substitution for Pb+2. Using ICP-MS we analyzed the [Cr] of 32 cerussite samples colored from clear to dark yellow and green. One dark yellow sample, obtained from the Australian Museum, is the original Petterd collection type locality. [Cr] of yellow cerussite is less than that required for color change in other minerals (e.g., 5000 ppm for ruby). Some yellow cerussite has [Cr] < [Cr] of clear and white cerussite. Yellow cerussite from the Tiger Mine, AZ, only contains 2.1 ppm Cr, which is < [Cr] of 8 of the 17 clear and white cerussite samples. Even the Petterd collection type sample of deep yellow cerussite from the Magnet Mine only has 22.86 ppm Cr. The data suggest that chromium substitution does not exclusively produce the yellow or green color observed in cerussite. We suggest that use of the term chrome cerussite be discontinued, and recommend that the descriptor yellow cerussite be used to describe this attractive yellow occurrence of cerussite. Work is ongoing to determine the exact cause of the yellow color in some cerussite.