Adventitiously introduced materials




Magnetite, iron, haematite and limonite. Scotland


Click hereA magnetic separate in which angular magnetite (brown grey), a coarse-grained crystal of haematite (blue-grey, centre top left) and limonite (blue-grey, low reflectance, centre bottom left) are natural phases. Tramp iron (white, high reflectance) occurs as long splinters or curved flakes (right centre). The grey matrix is resin.



Zircon, ilmenite, copper and copper oxides. Gold concentrate. Brazil


Click hereIlmenite (light brown-grey, bottom left, top left) and rounded to subhedral zircon (grey) are natural phases. An adventitious curved splinter of copper (pink, high reflectance) has oxidized to copper oxides and hydroxides (blue-grey, centre). Light blue areas with fine scratches surrounding ilmenite and zircon are relict carbon-coating. The grey matrix is resin.



Zircon, native gold, copper and iron. Gold concentrate. Brazil


Click hereRounded to subhedral zircon (grey) is accompanied by irregular-shaped high fineness gold (~960) (yellow, high reflectance) grains. Copper metal (pink, high reflectance, centre) and iron (white, high reflectance, top left) are rounded and are adventitious. Blue-grey areas are relict carbon-coating. Black areas are a polishing pit and an erupted air bubble in the resin (bottom left).



Bornite, chalcopyrite and chalcocite. Jersey, Channel Islands. Britain


Click hereA dried oil droplet (multicoloured, bottom right) is present within altered bornite. Although its colours are similar to those of tarnished bornite the circular shape is characteristic. Unaltered bornite is present in trace amounts and is extremely difficult to see (centre, top right). It is pink-brown and has a slightly lower reflectance than altered bornite (yellow-brown) which is the main phase. Altered bornite carries thin chalcopyrite (yellow, higher reflectance) and chalcocite (blue, lower reflectance) lamellae oriented along (111). Extensive alteration of this type has been called 'idaite' or interpreted as exsolution between chalcopyrite and bornite.



Niccolite, sphalerite and carborundum grit. Settlingstones Mine, North Pennines, Britain


Click hereCoarse crystals of niccolite (light brown-yellow, top) have partial rims of sphalerite (light grey, top). Fine-grained angular carborundum grinding grit (light grey, similar reflectance to sphalerite, centre left and right) occurs within the resin (dark grey, centre). Black areas are polishing pits.The restricted grain-size and angular nature of the grit are characteristic. It is often accompanied by angular sulphide fragments which have been plucked from the section.



Zircon, native gold and amalgam rim. Gold concentrate. Brazil


Click hereRounded to subhedral zircons (dark greys) show faint internal reflections about their margins. Native gold (yellow, centre) is enclosed within an amalgam rim (pale yellow-white, centre) which is still partially covered by carbon-coating (reddish areas). This amalgam rim could be confused with supergene gold rims but amalgam is whiter than the gold core rather than yellower (see Plate 45) as is the case for gold enrichment rims. Blue-grey areas around zircon and gold grains are relict carbon-coating.