At 02:45 PM 2000/04/26 -0400, Bob Tracy "mused":
>...
>... how sure can we be normally (75%, 90%, 99%?), or SHOULD we
>be, that we have correctly identified all minerals present in any
>given thin section? ...it seems to me we always have to live with the
>possibility that something has been missed (maybe something
>important) because it masqueraded petrographically as a more
>"expected" or common mineral.
Good example is aragonite in high P/T rocks, postulated thermodynamically by
Van Hise (1904), experimentally predicted by Jamieson (1953) Macdonald
(1956) and Clark (1957), but missed by dozens of highly competent
petrographers even though it clearly would have been important and should
have been "expected". Not until 1961 was metamorphic aragonite finally
identified, by Coleman and Lee in blueschists near Cazadero. Within a year,
aragonite had been identified in hundreds of thin sections from earlier PhD
studies at Berkeley and Stanford etc, and an important new experimental
study of the kinetics of aragonite=>calcite (Brown Fyfe&Turner 1962 J
Petrol) had shown that preservation of metamorphic aragonite during
exhumation depended on absence of H2O and geothermal gradient <12 degrees C.
Dugald
Dugald M Carmichael PhD PEng Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
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