Jung Ho Ahn

Hematite nanospheres of possible colloidal origin from a precambrian banded iron formation.

Exceptionally small spheres (nanospheres) of hematite (diameters between 120 and 200 nanometers) occur in the Marra Mamba Iron Formation of the Hamersley Basin, Australia. The nanospheres are clustered into small aggregates and may have formed by structural ordering and dehydration of colloidal iron hydroxide particles. Individual spheres consist of numerous thin, curved hematite platelets surrounding a central void that is approximately half the diamter of the sphere; this texture suggests that they formed by a volume reduction of the original colloidal particles by ∼12.5%. The occurrence of hematite nanospheres supports the hypothesis that some ofthe iron was deposited colloidally during the development ofbanded iron formations, approximately 2.5 billion years ago.

Transmission electron microscopy of native copper inclusions in illite

The occurrence of anomalous copper concentrations in biotites from rocks associated with porphyry copper deposits has long been of interest (Banks 1974; Rehrig and McKinney 1976; Hendry et al. 1981). Submicrometer particles of native copper were discovered in biotites through transmission electron microscope (TEM) and electron microprobe investigations (Ilton and Veblen 1988, 1993); such inclusions were produced experimentally using copper sulfate solutions at atmospheric temperature and pressure (Ilton et al. 1992). Since native copper inclusions are found only in altered sheet silicates, Ilton and Veblen (1988, 1993) concluded that the inclusions formed during weathering rather than during the hydrothermal events. Their findings suggest that sheet silicates act as copper sinks rather than as sources for primary mineralization (Titley 1988). Since sheet silicates other than biotite and phlogopite are also associated with copper deposits, a remaining question is whether only trioctahedral micas can be the hosts of copper inclusions. In the present study, we report the occurrence of submicrometer copper inclusions in weathered illites. Such inclusions suggest that a variety of sheet silicates other than trioctahedral micas can accommodate copper in their interlayers and could be useful indicators of Cu enrichment.