A supergene ore from a magmatic nickel sulphide deposit near Kambalda contains pyrite, violarite (FeNi2S4), chalcopyrite and millerite (NiS). Using the table of mean atomic numbers (Ž, in notes) and the formula for calculating Ž (mean Z, in notes) discuss the suitability of using an SEM/back-scattered electron (BE) system for carrying out mineralogical investigations of this ore. In your discussion, assume that there needs to be a difference of about 0.5 in
the mean atomic numbers between minerals, in order to distinguish them on a BE image.  If the mining company wishes to use image analysis to assist in flotation studies on the ore, what type of system would need to be used. Explain the
reasons for using this system. 

Table 2: Mean Atomic Numbers for Sulphides compared to Atomic Numbers of Pure Native Metals (from Reed, 1996).
.Sulphides
Native Metals
Pyrite
20.7
Rhodium
45
Chalcopyrite
23.5
Palladium
46
Bornite
25.3
Silver
47
Sphalerite
25.4
Osmium
76
Chalcocite
26.4
Iridium
77
Tetrahedrite
32.5
Platinum
78
Galena
73.2
Gold
79

Ž = a(A1Z1) + b(A2Z2)…/ ΣA where a, b etc represent atomic proportions of
elements in the mineral.
For example, Ž for chalcopyrite (CuFeS2) is
(29 x 63.54) + (26 x 55.85) + 2(16 x 32.06) / 183.51 = 23.5