Photomicrograph Tuesday – Pyrite
22. March, 2011 by Elli Goeke
I was sitting in a paleomagnetism talk at NE-NC GSA in Pittsburgh Sunday when the speaker put up a picture of several unidentified ore minerals & pyrite. 1) I had never seen a reflected light photomicrograph in a conference talk before [I don't normally sit in paleomag sessions...] and 2) pyrite is this week’s choice!
So, what does pyrite look like? In transmitted light, pyrite will be another one of the opaques (black both in PPL & XPL).
In reflected light, pyrite tends to be white to yellow-white to yellow in color. It can be difficult to distinguish from chalcopyrite (stronger yellow color in the images below):
The small differences in color are frequently difficult to distinguish from each other:
Coarse-grained pentlandite (yellow-brown, centre and right) shows characteristic cleavage along (111) and is intergrown with pyrrhotite (pink-brown, lower reflectance than pentlandite, left and bottom right). Pyrrhotite has rims of pyrite (white, centre bottom) and chalcopyrite (centre) which has tarnished to a dull yellow. Minor amounts of pyrite form areas within pyrrhotite along crystal boundaries (top left). Dark grey areas are silicates and black areas are polishing pits.http://www.smenet.org/opaque-ore/
The central spinel phase is zoned with a lower reflectance core (light grey, centre) of chrome-rich spinel (ferrochromite) and a higher reflectance iron-rich rim (magnetite) (bottom centre). It is extensively shattered and the fractures are infilled with chalcopyrite. Chalcopyrite (yellow, left) is intergrown with pyrite (pale yellow-white, higher reflectance, right) and euhedral to subhedral highly altered pentlandite (light yellow brown, many polishing pits, top centre). Violarite (brown, top right) is the main alteration product of pentlandite. Dark grey areas are silicates. http://www.smenet.org/opaque-ore/
Pentlandite (light brown, centre) is intergrown with chalcopyrite (yellow, right), pyrite (pale yellow-white, centre bottom) and minor amounts of pyrrhotite (lilac-grey, centre right). Silicate gangue (grey) shows internal reflections. Black areas are polishing pits. The sulphides are interstitial to the silicates. http://www.smenet.org/opaque-ore/
I think the take-home message here may simply be: either get very good at distinguishing small changes in color & how reflective a surface is OR hope that your department has an SEM down the hall. At Gustavus, we’re dealing with the former right now.
Next week’s mineral will be one that comes up time & time again here on my blog: garnet