Simonson is an outstanding petrographer and if it

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square-tipped terminations” of radiating aragonite crystals. Simonson is an outstanding petrographer and if it worries him, it worries me. The large crystal fans described and illustrated by Sumner and Grotzinger (2000) are also very similar in scale and morphology to the Solfifera radiating prismatic gypsum. The interested reader should compare 3A of Sumner and Grotzinger (2000) with Figure 23 in Hardie and Eugster (1971) and Sumner and Grotzinger’s (2000) Figure 8A with Figures 4, 5, and 6 in Hardie and Eugster (1971). Furthermore, Sumner and Grotzinger (2000, p. 134) pointed out that in the 2.7 Ga Steeprock Group of Ontario, Cana- da, Hofmann (1971) noted the presence of large radiating pseudomorphs that he suggested were originally either gypsum or aragonite. Sumner and Grotzinger (2000, p. 134) pointed out that later authors such as Walter (1983) and Wilks (1986) “favored only the gypsum interpretation, on the basis of the large size of the crystals, which extend for up to 25 cm.” There are other features that support an evaporitic environment for both the Campbellrand-Malmani carbonates and the Carawine Dolomite. Both Sumner and Grotzinger (2000) and Simonson et al. (1993) report “halite pseudomorphs,” clear evidence for highly evaporative conditions. Indeed, Simonson et al. (1993) have identified dolomite pseudomorphs after gypsum associated with halite molds. It must be pointed out that whether we are considering periods of “calcite seas” or “aragonite seas,” gypsum will precipitate from evaporating seawater after the CaCO 3 stage but before halite precipitation. Where there was halite, there also will have been gypsum. In another direction, Simonson et al. (1993) report “zebraic dolomite cement” pseudomorphous after calcite in the Carawine Dolomite. Fur- thermore, they illustrate in their Figure 12 an oolitic arenite with geopetal matrix and dolomite spar filling the pore spaces between the ooids, these latter in turn show the characteristic radial fabric of primary calcite. Rel- evant to this feature is Sandberg’s (1975, Fig. 16) illustration of a Jurassic calcite ooid with a classic radial fabric typical of “calcite sea” ooids that had nucleated on a gastropod aragonitic shell, which in turn has been re- placed by blocky calcite spar. In this regard, Sandberg (1975, p. 532) notes that “a marked difference in textural preservation exists between calcite- replaced ancient shells known to have been aragonite and coexisting ooids presumed by analogy with modern equivalents to have been originally aragonite. Since organic matrix and paramorphic replacement arguments are untenable in this regard, no justifi cation remains for ascribing an origi- nal aragonitic mineralogy to ancient calcitic ooids that retain fine texture. Those ooids must have been originally calcite.” In the sedimentological literature there are many well-illustrated examples of calcitic ooids with this primary radial fabric. Their occurrences dominate the periods of “cal- cite seas” during the Phanerozoic.
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  • Winter '08
  • Barfod
  • Calcite, Dr. Sumner, aragonite pseudomorphs, Precambrian aragonite seas, aragonite precipitation

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