I previously posted two examples of bornite, a hand specimen from my mineral collection and some photos of disseminated bornite within chalcopyrite and other sulphides from diamond core taken from 339.4m down hole at the Monty deposit in WA.
I was a bit curious to understand why the photos of the bornite in the core in PXX’s hole ZX-18023 are reddish pink as bornite is know to be reactive and change colour quickly to be multicoloured (blue, purple and red like in my photos) when exposed to air. It’s also curious as the core from the photo is from relatively shallow depth (from 26.6m) where some oxidisation might be expected.
I found a scientific study that measured various samples of bornite to see how quickly the reflectance from it’s surface changes over time when it is exposed to air and oil (for different wavelengths of light).
Figure 2 shows that in one sample of bornite tested the colour change (change in reflectance at a wavelength of 600nm) is logarithm with time and very rapid, with colour change occurring on time scales of a few minutes. 600nm is the wave length of light that we see as orange (between yellow and red). The percentage of reflectance from the surface of this sample changed by 6.5% in 5.5 minutes at that wavelength.
The study also found that other samples would take a few years to reach the level of tarnish that some samples would achieve in a few months, which they put down to these probable reasons.
“Several factors may affect the rate of tarnish at constant temperature and humidity, among them (i) the associated minerals--qualitative observations on bornite with oriented lamellar inclusions of chalcopyrite coexisting with inclusion-free bornite show that the latter tarnishes more rapidly; (ii) grain size and orientation - granular aggregates with grains in random orientation are observed to tarnish at variable rates, and, in general, the smaller the grain size the slower the tarnish formation; (iii) the nature of the polished surface of the bornite - poorly polished surfaces tarnish more rapidly, due probably to the greater surface area exposed to the atmosphere, and (iv) chemical composition - the data presented in this work show that the cation- deficient bornite tarnished more slowly than with a slight cation excess; many of the so- called 'orange' or 'anomalous' bornites in the literature are cation-deficient (Sillitoe and Clark, 1969; Kosyak, 1969) and their orange appearance may be the result of the retarded tarnishing effect relative to stoichiometric or 'normal' bornite.“
So there you have it on the colour, this is probably not “normal” bornite IMO, if anyone was wondering.
In anycase, I’ll stick with, there is not enough of it there in that interval to be payable Cu , no matter the colour. Time will tell who is right or wrong when the assays result get published.
