Saponite from Moldova Noua, Romania

Abstract

SAPONITE FROM MOLDOVA NOUA, ROMANIA Argilization associated with hydrothermal activity in the vicinity of a 'porphyry copper' deposit near Moldova Noun, western Romania, is represented mainly by trioctahedral clay minerals. These include saponite, hectorite, stevensite, sepiolite, chlorite, vermiculite, kerolite, talc, and members of the serpentine group (Ianovici & Neac~u, 1970; Neac~u, 1970). Some saponite formed by direct crystallization from hydrothermal solutions but it occurs mainly as pseudomorphous replacements of grossularite and diopside, and less commonly replacing serpentine minerals and sepiolite; it is frequently associated with calcite. Chemical analyses of two samples of saponite are given in Table I and the corresponding structural formulae are given below: sample 4206 (Si~.35Tio.olAlo.64)(Mgs.44Alo.27Fe3+o.2oMno.ol)O2o(OH)4(C ao.39Nao.01), nH20 sample 4261 (SiT.35Tio.03Alo.62) (Mgs.~2Alo.loFe3+o. 12Mno.ot)O20(OH)4 (Ca0.31Nao.02Ko.o~). rt H20 Cation exchange capacities, determined by NH4-exchange, and constituents of the original exchangeable cation assemblage are shown in Table 2. Calcium is the dominant exchangeable cation in the natural clays. The saponites from Moldova No~a show both lower Al-for-Si substitution in the tetrahedral sheet and lower Al/Fe-for-Mg substitution in the octahedral sheet than most saponites described in the literature. However, the octahedral cation totals of 5.85 and 5.95 are normal for trioctahedral smectites and the interlayer cation charges of 0.39 and 0.40 are are normal for saponites. Under the microscope the saponite occurs as colourless crystals 0.001-0.14 mm (average 0-01 mm) in size. Refractive indices and birefringence have the following values: n v-n~ = 1.500-1.485 = 0.015. The DTA curve shows an endotherm at 150~ resulting from loss of interlayer water,. followed by a smaller endotherm at 260~ due to loss of hydration water of the Ca 2 § cation (Mackenzie, 1957). No further reactions are noted on DTA, DTG and TG curves until ~840~ At this temperature structural water is lost, in common with saponites from other localities (Mackenzie, 1957; Todor, 1972). XRD patterns are shown in Fig. 1. Small peaks at 3.03 and 3.34 A are due to the presence of calcite and quartz impurities. The main difference between XRD traces of saponites from Moldova Nou~i is in the position of the d(060) reflection, which varies in the range 1.525-1.533 A. This variation is due mainly to Al-for-Si substitution in the tetrahedral sheet and, to a lesser extent, to substitutions within the octahedral sheet.