Crystallographic evidence of sulfate evaporites in the Kromberg Formation, Onverwacht Group, South Africa
The 3.5 Ga Kromberg Formation of the Onverwacht Group, Barberton Greenstone Belt, South Africa, includes a basal silicified shallow-water evaporite sequence deposited in a series of small fault-bounded basins. Other early Archean replaced and recrystallized evaporites have been interpreted to have included both primary gypsum and aragonite based on general crystal morphology and the overall similarity of these units to Phanerozoic evaporites. Crystal interfacial angle measurements, however, have not been widely reported to support interpretations of primary mineralogy. Although the primary evaporite minerals in the Kromberg Formation have been replaced by chert and druzy megaquartz, interfacial angles of silicified pseudohexagonal, prismatic to bladed crystals can be measured with a contact goniometer. Measurement of angles between the six prism faces display a biomodal distribution about 55/sup 0/ and 69/sup 0/. Within individual crystals, these angles show a general arrangement of two at 69/sup 0/ separated by sets of two angles at 55/sup 0/. These relationships closely resemble those of gypsum which has four angles of (010) (120)=55.75/sup 0/ and two of (120) (120)-68.50/sup 0/. No evidence has been found for the existence of aragonite which has six angles of (110) (110)=60/sup 0/. Textural and geometric crystal comparisons of these silicified crystals with Messinian algal-related selenite and modern gypsum support the proposal of a gypsum precursor. The common presence of gypsum in this and other greenstone belts suggests that the early Archean ocean may have been at or near saturation with respect to sulfate.
- Research Organization:
- Louisiana State Univ., Baton Rouge (USA)
- OSTI ID:
- 6643854
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
EVAPORITES
CRYSTALLOGRAPHY
MINERALOGY
SEDIMENTARY BASINS
SOUTH AFRICA
ARAGONITE
CHERT
GEOLOGIC FORMATIONS
GYPSUM
AFRICA
ALKALINE EARTH METAL COMPOUNDS
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CALCIUM SULFATES
CARBON COMPOUNDS
CARBONATE MINERALS
CARBONATES
GEOLOGIC STRUCTURES
MINERALS
OXYGEN COMPOUNDS
ROCKS
SEDIMENTARY ROCKS
SULFATE MINERALS
SULFATES
SULFUR COMPOUNDS
580300* - Mineralogy
Petrology
& Rock Mechanics- (-1989)