New occurrence and characterization of Ni-serpentines in the Kwangcheon area, Korea

Abstract

A B S T R A C T:Pecoraite and nepouite, Ni-serpentines, occur in the serpentinized ultrarnafic rocks in the Kwangcheon area, Korea, where the parent rock is classified as harzburgite and/or lherzolite. Pecoraite was precipitated twice from the solution; the early-formed pecoraite coexists with magnetite, millerite, and polydymite both in the Buk-and Nam-sites, while the late-formed pecoraite appears as well-grown colloform and opaque-free phase only in the Buk-site. The typical colloform texture of the late-formed pecoraite strongly indicates that it was precipitated from the solution in supergene conditions. Pecoraite is characterized by its extremely high Ni content and the difference in Fe content between the early-and late-formed pecoraite. Nepouite is distinguished from pecoraite by its prismatic morphology and the large degree of isomorphous substitution between Ni and Mg. The phase relations among coexisting magnetite-millerite-polydymite assemblage with the earlyformed pecoraite suggest that the pecoraite might have precipitated in the extremely limited fo2 and .fs2 environment from the highly Ni-concentrated solutions and is stable at 25~ and 1 bar.

Additional data on reevesite and its Co-analogue, as a new member of the hydrotalcite group

“…The Ni sulphides occur less frequently in the Buk-site than in the Nam-site. In addition, the relative proportion of the millerite is higher than that of polydymite, especially in the Buk-site (Song et al, 1995). This occurrence is closely comparable with those of the Wolf Creek Meteorite (White et al, 1967) and the nickel ore from the Ben Accord area (De Waal & Viljeon, 1971).…”

“…sulphate forms of these minerals. Therefore, the substitution of SO]-for CO 2-, in spite of its low proportion, indicates the existence of significant amounts of SO 2-, possibly provided from the oxidation of pre-existing Ni sulphides (Song et al, 1995).…”

“…The reevesite occurs as golden yellow fine-grained aggregates replacing early-formed pecoraite-magnetite-millerite-polydymite assemblages during the advanced weathering processes in the Buk-site of the Kwangcheon area (Song et al, 1995). As the reevesite is mixed with these pre-existing minerals and is fine-grained, it is very difficult to separate cleanly from othe material in the samples.…”

“…It is composed of positively charged brucite-like sheets and contains chargecompensating CO3 z-anions and water molecules in the interlayers, which identify it as a member of the hydrotalcite group. The reevesite found in the serpentinized ultramafic rocks of the Kwangcheon area in Korea is characterized by pseudomorphic textures after a pecoraite-magnetite-milleritepolydymite assemblage (Song et al, 1995) and the significant substitution of Co 3+ for Fe 3+ in the octahedral sites. Piret & Deliens (1980) described comblainite as the Co-analogue of reevesite in the hydrotalcite group.…”

Serpentine and chlorite as effective Ni-Cu sinks during weathering of the Aguablanca sulphide deposit (SW Spain). TEM evidence for metal-retention mechanisms in sheet silicates

“…This, together with the textural and compositional features observed, suggests a supergene origin for the nepouite. This could have precipitated from groundwater solutions or, perhaps, be derived from the solid transformation of chlorite through a late Ni-enrichment process, as has been reported for some serpentine minerals involved in the formation of garnierite (e.g., Trescases, 1975;Milton et al, 1983;Song et al, 1995;Tauler et al, 2007).…”