Motukoreaite, a new hydrated carbonate, sulphate, and hydroxide of Mg and Al from Auckland, New Zealand
1977; Cambridge University Press; Volume: 41; Issue: 319 Linguagem: Inglês
10.1180/s0026461x00038147
ISSN1471-8022
AutoresK. A. Rodgers, J. E. Chisholm, Robert J. Davis, Campbell S. Nelson,
Tópico(s)Paleontology and Stratigraphy of Fossils
ResumoMotukoreaite occurs as relatively abundant, white, clay-like cement in both beach-rock and basaltic volcanic tuffs on the flanks of a small, extinct, late Pleistocene, basaltic cone at Brown's Island (Motukorea), within Waitemata Harbour, Auckland, New Zealand (36° 50′ S., 174° 35′ E.). The occurrence was originally recorded by Bartrum (1941) as ‘beach limestone’ found at two places of the island's shore. The beach-rock consists of a grain-supported fabric of poorly sorted, well-rounded, alkali-olivine basalt pebbles and granules, subangular to sub-rounded fresh olivine sand and abraded sand- and gravel-sized bioclasts in a colourless to pale yellow-green aphanocrystalline matrix of motukoreaite. Additional detritals include quartz, feldspar, and sedimentary rock fragments. Stereoscan examination of the surface of pieces of the cement prised from the beach-rock showed a box-work of plate-like crystals with a hexagonal form in which individuals measured about 3×3×0·02 microns (fig. 1). Wet-chemical analysis of a separate of the cement containing some 5 % quartz and traces of calcite and goethite gives SiO 2 5·55, Al 2 O 3 17·87, Fe 2 O 3 0·73, CaO 0·92, MgO 22·98, MnO 0·70, ZnO 0·56, Na 2 O 0·71, K 2 O 0·10, CO 2 9·32, SO 3 10·00, H 2 O+ 19·62, H 2 O- 10·35, sum 99·41 %. The unit-cell formula using obtained unit-cell constants and measured specific gravity 1·43) is (Na 0·73 K 0·07 ) ∑0·80 (Mg 18·13 Mn 0·32 Zn 0·21 ) ∑18·66 Al 11·15 (CO 3 ) 6·22 (SO 4 ) 3·97 (OH) 51·19 27·20H 2 O. Of several idealized formulae that may be proposed NaMg 19 Al 12 (CO 3 ) 6.5 (SO 4 ) 4 (OH) 54 ·28H 2 O is preferred.
Referência(s)