2020

Colourless Diopside from Kenya and Canada

first published in Facette 22 (February 2016)

Figure 1: Samples A-C are from Kenya, sample D-E from Canada. Photo: M. S. Krzemnicki, SSEF

Diopside, a calcium magnesium clinopyroxene (MgCaSi2O6), an end-member of the diopside-hedenbergite series (pyroxene) is a rather rare gem species in the trade. Commonly found in greenish brown stones (mostly due to iron content) to dark brown cabochons with an oblique star effect, its most attractive variety is emerald-green Cr-diopside from Russia (Ural) and a few other localities, owing to its attractive colour to the incorporation of traces of chromium. Gem-quality colourless to near colourless – thus chemically rather pure – diopside has already been described from the Mogok Stone Tract (Themelis, 2008), Tanzania (Milisenda & Wehr, 2009) and Canada (Robinson & Wight 1997).

Recently, the SSEF analysed three small samples from Kenya with a very subtle yellowish tint due to minor iron (A: 0.40 wt% FeO, measured by ED-XRF) and two absolutely colourless samples from Canada (Figure 1). Raman analyses revealed for all five samples distinct vibrational peaks, perfectly fitting with our diopside references. Vanadium and chromium, both responsible for greenish to intense green colours, were below the detection limit in our studied samples.

Figure 2: The picture above shows the reaction of the 5 samples under LWUV, where the three Kenyan samples show no reaction. The lower picture shows the reaction under SWUV. Photo: M. S. Krzemnicki, SSEF

Interestingly, only the diopside from Canada showed a distinct orange reaction under long-wave ultraviolet light, whereas the samples from Kenya remained inert (Figure 2). When exposed to shortwave ultraviolet, all investigated samples showed an equally bluish white reaction. This observation of varying UV reactions of diopside is not new and has already been reported in literature (Henkel 1988 – 1989, Robinson & Wight 1997). In our case, however, this property allowed a quick separation of diopside from Kenya and Canada.