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Plumboan crichtonite from the Fazenda Guariba, Minas Gerais, Brazil
Mineralogical Record, Mar 1994 by Foord, Eugene E, Chaves, Mario Luiz da Sa C, Lichte, Frederick E
Senaite, Pb(Ti,Fe,Mn) sub 21 O sub 38 , a member of the crichtonite group, was first described by Hussak and Prior (1896, 1898) from alluvial gravels near Dattas, Minas Gerais, Brazil. It was subsequently reported (always from alluvial stream gravels) from about seven additional localities (see Cassedanne, 1986, for details and references). Foord et al., (1984) studied Zn-bearing senaite from St. Peters Dome, El Paso County, Colorado, and a specimen (DGM/DNPM no. 1651) from Dattas, Diamantina region, Minas Gerais, Brazil. Senaite is a very rare mineral, having been found in less than ten localities worldwide: (1) Brazil, (2) Nezilovo, Macedonia (Vujanovic, 1970), (3) Alinci, Yugoslavia (Bermanec et al., 1992), (4) Switzerland (Stalder and Buhler, 1987). (5) Italy (Armbruster and Kunz, 1990), (6) Colorado (Foord et al., 1984) and (7) an unusual rhenium-rich (2.6 wt. % ReO sub 2 ) member of the crichtonite group from an unknown location (Sarp et al., 1981). Four grains of specimen DGM/DNPM 1651 proved to be senaite with about 0.5 wt. % ZnO, however, a fifth grain contained 7.7 wt. ZnO (Foord et al., 1984).
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Cassedanne (1986) has described "senaite" from the first in situ source in Brazil, at the Fazenda Guariba. south of the village of Presidente Kubitschek, 30 km south of Diamantina, Minas Gerais, Brazil. Only a semiquantitative emission spectrographic analysis was done by Cassedanne; quantative results from this study show the material to be plumboan crichtonite rather than senaite. Many Brazilian mineral dealers have had material from the Fazenda Guariba for sale and have labeled it "senaite" according to Cassedanne's identification.
Details of the geology and access are given by Cassedanne (1986) and Chaves et al. (1988). Specimens of the "senaite" were provided to the senior author by Carlos do Prado Barbosa and the second author for additional study. Approximately 20 crystals of "senaite" from the Fazenda Guariba were examined chemically and all were found to be plumboan crichtonite rather than senaite.
RESULTS
Complete chemical analyses of the crichtonite (Table 1) were done by electron microprobe and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) methods. (Table 1 omitted) Electron microprobe analyses were done using an ARL SEMQ instrument, 15 kV, 10 nanoamps sample current on a Faraday cup, 10 micron beam diameter, with 20 second count times on peak positions and 10 second count times on background positions. The rare-earth elements were not determined by electron microprobe because of potential interference problems. All other elements greater than atomic number 9 were measured by both electron probe and LA-ICP-AES methods. Probe standards included synthetic TiO sub 2 for Ti. synthetic Fe sub 2 O sub 3 for Fe, synthetic SrBaNb sub 4 O sub 10 for Sr and Ba, natural garnet for Mg, Mn, Ca, Si and Al, chromite for Cr and PbS for Pb. A Fisons VG PlasmaQuad 2+ instrument equipped with a Nd/YAG laser operated at 1064 nm was used for the LA-ICP-MS analysis of the crichtonite. The iron value determined from the electron microprobe analysis was used as a standard for which to compare all of the other elements determined. All elements in the periodic table were looked for except the noble gases, H, C, N and O. Ferrous iron was not determined for this crichtonite. An empirical formula, based on 38 oxygen atoms and all Fe as ferric iron, is as follows: (Formula omitted) The cation sum of the octahedral cations is less than the ideal 21. This feature has been reported previously (e.g. Gatehouse et al., 1979) for members of the crichtonite group and is believed to be real based on the number of such reported occurrences and the fact that virtually all elements were looked for in the currently studied plumboan crichtonite. Figure 1 is a CNR (chondrite-normalized ratio) plot of the REE distribution for the crichtonite. (Figure 1 omitted). There is a distinct double-hump distribution pattern, i.e. a peak for the LREE's (light rare earth elements) and another for the HREE's (heavy rare earth elements). Maxima are at La and Dy. This double-hump pattern, observed in other members of the crichtonite group as well, is evidence that there are two REE sites present, large and small respectively (Gatehouse et al., 1979; Foord et al., 1984).
Unit-cell refinement of the plumboan crichtonite was done using an automated Siemens D500 diffractometer, Cu K alpha radiation (1.54178 angstrom), zero background quartz plate, NBS SRM 640 silicon as an internal standard, 20 second count times and 0.02 deg step increments. The refinement using the unit-cell program Job 9214 (Appleman and Evans, 1973) yielded a- 10.376(2) angstrom, c- 20.967(4) angstrom, V = 1955.0(7) angstrom. These cell dimensions agree well with those for other specimens of crichtonite and senaite (see Fig. 2). (Fig. 2 omitted)
The specific gravity of an unaltered grain, determined on a Berman microbalance, is 4.60(2). D sub calc using the determined cell volume and molecular weight is 4.56 g/cm sup 3 . Lower specific gravities were measured on other grains and are due to incipient alteration and hydration.
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