Hawk’s-Eye Quartz Sold as Vivianite
A New Source of Garnets with Directional Color Shift
shifts to 3348 cm, and its full width at half maximum (FWHM) increases from 11 to 57 cm. Based on these parameters, the Raman spectra of the present two samples indicate that they have low F content and therefore consist of montebrasite. This is also consistent with the relatively high RI values that were measured for both stones (cf. Schnellrath et al. 2010). Raman spectroscopy of the two-phase inclusions in both stones detected CH, and N, in the gas bubbles (cf. Yu et al. 2021), but we were unable to identify the fluid phase.
Gem-quality montebrasite is rarely seen in the gem market. Large pale yellowish green gemstones are known from Brazil (Schnellrath et al. 2010), pale purple gems have been documented from Afghani- stan (Laurs et al. 2018) and blue/white material was recently found in Rwanda (Vertriest et al. 2023).
Tiantian Huang and Yujie Gao
peter.gao@guildgemlab.com
Guild Gem Laboratories
Shenzhen, China
References
Choudhary, G. 2015. Gem News International: A remarkably large amblygonite-montebrasite carving. Gems & Gemology, 51(1), 98-99.
Montebrasit in Edelsteinqualitat: ein seltener Fund aus Brasilien [Gem-quality montebrasite: A rare find from Brazil]. Gemmologie: Zeitschrift der Deutschen Gemmologischen Gesellschaft, 59(3—4), 95-102.
Laurs, B.M., Falster, A.U. & Simmons, W.B. 2018. Gem Notes: Montebrasite from Afghanistan. Journal of Gemmology, 36(4), 286-287.
Vertriest, W., Yuda, G. & Henley, J. 2023. Gem News International: Blue amblygonite-montebrasite from Rwanda. Gems & Gemology, 59(4), 524-525.
Rondeau, B., Fritsch, E., Lefevre, P., Guiraud, M.., Fransolet, A.M. & Lulzac, Y. 2006. A Raman investigation of the amblygonite-montebrasite series. Canadian Mineralogist, 44(5), 1109-1117, https://doi.org/10.2113/gscanmin.44.5.1109.
Yu, Y., Hu, W., Chou, I.M., Jiang, L., Wan, Y., Li, Y., Xin, Y., Wang, X. et al. 2021. Species of sulfur in sour gas reservoir: Insights from in situ Raman spectroscopy of S-H,S—CH,-H,0 system and its subsystems from 20 to 250°C. Geofluids, 2021, article 6658711, https://doi.org/10.1155/2021/6658711.
Schnellrath, J., Scholz, R., Krambrock, K., Milisenda, C.C. & de Figueiredo Epaminondas, A.M. 2010.
Hawk’'s-Eye Quartz Sold as Vivianite
A parcel of loose cabochons represented as vivianite was purchased by Gemological Science Interna- tional’s (GSI) laboratory at the Tucson gem shows in 2018. The stones weighed 3-6 ct and were dark greyish blue and semi-translucent to opaque with a fibrous appearance (e.g. Figure 19).
According to mineralogy textbooks (e.g. Anthony et al. 2003), vivianite is a hydrated iron phosphate mineral, Fe2(PO,), 8H2,O, named in honour of John Henry Vivian (1785-1855), an English mineralogist and mine owner who first discovered the mineral in Cornwall, south-west England. It often displays a bladed or fibrous structure and occurs in various geological environments such as the oxidised zone of metallic ore deposits and complex granitic pegmatites, and may even be found replacing organic material in fossil bones. Vivianite is colourless to very pale green or blue before being exposed to light, which causes it to become dark greenish blue, blue or black due to the oxidation of iron from Fe2+ to Fe3+ (Watson 1918). In addition, vivianite is soft (Mohs 1'—2) and fragile (with perfect cleavage), posing challenges during lapidary work. Therefore it is most often appreciated in its rough state as a collectable mineral.
Figure 19: This 3.00 ct cabochon was included in a parcel sold as vivianite, but proved to be hawk’s-eye quartz. Photo by Prashant Bhosale, © GSI.
Two dark greyish blue semi-translucent cabochons that weighed 2.60 and 3.00 ct (one of which is shown in Figure 19) were selected from the parcel for testing, and compared to a reference sample of rough vivianite that was purchased at the Tucson gem shows in February 2023 (Figure 20). The cabochons had a fibrous appearance and appeared to display some chatoyancy when examined with pinpoint illumination. They had RI values of 1.544—1.550 and a hydrostatic SG of 2.66. By comparison, the values for vivianite listed by Anthony et al. (2003) are higher (n, = 1.579—1.616, ng = 1.602—1.656 and n, = 1.629-1.675, and SG = 2.68). Microscopic examina- tion of the cabochons revealed a fibrous appearance and fractures, whereas our reference sample of vivianite contained numerous black needle-like inclu- sions, step-like fractures and displayed strong blue pleochroism. Given the differences in appearance, as well as the low RI values and vague chatoyancy shown by the cabochons, we grew suspicious that they might not be vivianite.
Raman spectroscopy of the cabochons confirmed our suspicions by revealing a peak at 465 cm”! indic- ative of quartz. In addition, chemical analysis by EDXREF spectroscopy showed Si as the main element present, also consistent with quartz. By comparison, Raman spectroscopy of our reference sample matched the spectra of vivianite in the RRUFF database, and EDXRF analysis showed the expected Fe and P as the main elements.
Figure 20: vivianite specimen measuring 177.46 x 19.23 x 11.90 mm was used as a reference sample in this study. Photo by Prashant Bhosale, © GSI.
The fibrous appearance and dark greyish blue colour of the quartz cabochons are consistent with hawk’s-eye, which consists of quartz and blue crocid- olite (fibrous amphibole) that is inferred to have formed by a crack-seal process and is the precursor to yellow-to-brown tiger’s-eye (Heaney & Fisher 2003).
Deepa Srinivasa (deepas@gemscience.net) GSI, Mumbai, India
References
Anthony, J.W., Bideaux, R.A., Bladh, K.W. & Nichols, M.C. (eds) 2003. Vivianite. In: Handbook of Mineralogy. Mineralogical Society of America, Chantilly, Virginia, USA, https://www.handbookofmineralogy.org/pdfs/vivianite.pdf accessed 29 May 2024.
Heaney, P.J. & Fisher, D.M. 2003. New interpretation of the origin of tiger’s-eye. Geology, 31(4), 323-326, https://doi.org/10.1130/009 1-7613(2003)031 0323: NIOTOO 2.0.CO;2.
Watson, T.L. 1918. The color change in vivianite and its effect on the optical properties. American Mineralogist, 3(8), 159-161.
Rhodonite from Slovakia
Rhodonite typically occurs as a massive opaque mineral aggregate with a distinctive rose-pink colour, and is a popular ornamental gem material. Despite its resemblance to other pink gems such as rhodochrosite, rhodonite’s resistance to acids and its association with black Mn oxidation products help distinguish it. Rhodonite and pyroxmangite are both polymorphs of CaMn3;Mn(Si,;0,;), and are often found together in various ore deposits. Gem-quality rhodonite is sourced from locations worldwide, including Australia, Brazil, Canada, Indonesia, Iran, Italy, Japan, Peru, Russia, Sweden, Switzerland and the USA (Diella et al. 2014; Rahinzadeh & Hadipanah 2020; Jutras 2023).
Although rhodonite suitable for jewellery use has not been previously reported from Slovakia, the mineral is associated with various metamorphosed Mn occurrences in the Slovak Ore Mountains (Slovenské Rudohorie) in the KoSice Region of eastern Slovakia. This region has undergone two tectono-metamorphic events under greenschist facies metamorphic condi- tions (Bajanik et al. 1983). At the Cierna baiia (Black mine) in Cuéma, Mn mineralisation is hosted by Early Paleozoic metacarbonate bodies situated within black phyllite and lydite (a silica-rich sedimentary rock), while at Prakovce the Mn occurrences are hosted by magnetite lenses in metarhyolite and tuff. At Poraé, rhodonite occurs in Early Paleozoic rocks consisting mostly of metachert lenses in metabasalts and mafic metavolcanic units. The classical mineralisation at