Colour zoning in Poona emerald crystals is rare but observed [
4,
15]. In our specimens, it is generally absent optically, both in hand specimen and plane polarized thin section petrography. In transmitted light, emerald in the pegmatite adjacent to the phlogopite schist displays rare zoning under crossed nicols (
Figure 2). More pronounced zoning is observed during cathodoluminescence (CL) studies with the zonation corresponding to changes in chemical composition (
Figure 3;
Table 1). Quartz in our study displayed no optical or CL zoning. Cores of some emerald show resorption and subsequent reprecipitation of additional growth zones, indicating multiple emerald mineralizing events consistent with [
15]. Electron microprobe analyses were conducted traversing multiple growth zones across the emerald crystal shown in
Figure 2, perpendicular to the growth zones and the c-axis of the crystal. Cr, V, and Fe concentrations determined by electron microprobe varied from one growth zone to another with the outer zones often enriched in Cr and to a lesser extent V and Fe (
Figure 3) showing decreased CL emission. However, the core of the emerald is also dark, indicating that other elements may also be responsible for CL emission or quenching. Interestingly the intensity of CL emission from the Poona emerald is generally the opposite of what is normally observed in emerald with the Cr and V enriched zones generally being more CL active (cf. [
25]). There is a good agreement between Cr and V concentration with a relatively poor correlation between these elements and Fe. Overall, the Poona emerald is chromium dominant, with the colour zones in the emerald crystal resulting in the observed colourless beryl cores and green emerald outer zones. A similar trend, but also the reverse trend of green (Cr-rich) cores enclosed by outer zones of colourless beryl has been documented [
4]. Since substitution occurs at the Y site in the crystal structure of beryl, the variations in colour are attributed to complicated substitutions at this site and/or slight variations in fluid composition, with values of Cr
2O
3, V
2O
3, and Fe
2O
3, ranging up to 0.27, 0.04, 0.28 wt % respectively. The concentrations of Fe, Cr, and V have been plotted relative to emerald analyses worldwide (
Figure 4). The elemental concentrations from this study and past data [
3,
4,
16] are in good agreement. However, as noted by previous researchers, emerald from these deposits is zoned and variable in chemical composition. The more chromium-rich zones of the crystal are plotted in dark and the colourless beryl in light on
Figure 4. It is easily identifiable that the concentrations of Cr have resulted in this spread in the data. Notably, the Poona emerald displays chemical overlap with deposits in Madagascar, Afghanistan, Colombia, and other Australian emerald deposits. Selected cation concentrations relative to Al and Mg + Mn + Fe respectively are plotted in
Figure 5 and
Figure 6. There is a difference between the colour zonations highlighted by the cation concentrations versus Al, as substitution occurs at this site with varying compositions of chromium and vanadium. The Na
2O values from this study range from 0.21 to 0.37 and slightly lower, but in agreement with previously published electron microprobe data [
16], with values ranging up to 0.49 and one anomalous value at 0.89 wt % Na
2O. Electron microprobe data from another study [
3] reported one Na
2O value of 0.48 wt %. Na concentrations are important as they are proportional to water contents within emerald [
26].
Previous mass-spectrometric studies of fluids trapped within the channels in beryl and within fluid inclusions have been used to characterize the quantitative and qualitative compositions of fluids within emerald [
26,
28,
29]. Water within the structural channels of emerald has been characterized as Type I and Type II, based on the orientation of the water molecule within the c axis [
30]. The Type I water is free of alkalis and unbonded. Type II water is coordinated to alkali cations: Na+, K+, Rb+ and Cs+ with two water molecules bonded to a Na+ ion. Depending on the concentration of Na+, enrichment in molecular H
2O can be predicted. Earlier studies [
6,
26] fit existing data to equations:
respectively with H
2O and Na
2O measured in wt %.