IMA 2009-056 = ferhodsite

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Marco E. Ciriotti
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IMA 2009-056 = ferhodsite

Messaggio da Marco E. Ciriotti » mer 02 mag, 2018 16:42

Referenza:
▪ Begizov, V.D. & Zavjalov, E.N. (2016): Ferhodsite (Fe,Rh,Ir,Ni,Cu,Co,Pt)9–xS8 — new mineral from Nizhny Tagil ultramafic complex. Novye Dannye o Mineralakh (New Data on Minerals), 51, 8–11 (in Russian with English abstract).

Abstract:
The new mineral ferhodsite (IMA 2009-056), ideally (Fe,Rh,Ni,Ir,Cu,Co,Pt)9–xS8 (abstract) or (Fe,Rh,Ir,Ni,Cu,Co,Pt)9–xS8 (title and text) was discovered in mineral separates from dunite of the Solov’yova Gora (central part of the Nizhniy Tagil ultramafic massif), Urals, Russia, and in a heavy mineral concentrate from the Konder placer deposit, Ayano-Mayaskiy region, Khabarovskiy kray, Russia.
The mineral is usually observed as isometric or platy, triangular, and corroded grains located along periphery of isoferroplatinum grains. Commonly, the new mineral is mantled and corroded by chengdeite and a solid solution that is intermediate in composition between isoferroplatinum and tetraferroplatinum. Other minerals observed in intergrowths with ferhodsite are cooperite, cuproiridsite, bowieite, and kashinite (laurite and erlichmanite are also mentioned, but only in the English version of the abstract). Opal commonly penetrates along well-developed {111} cleavage fractures in ferhodsite grains varying in size from a few to 100 μm with a mean of 30–40 μm (text); size of grains 10–50 μm and size of aggregates 40–70 or 100 μm (abstract). Ferhodsite has a black streak, metallic luster, in reflected light opaque, and light brownish-gray (English abstract) but also “grayish-creamy with a brownish tint” in Russian text. The micro-indentation hardness VHN20 = 516(15) kg/mm2. The density was not measured; Dcalc = 7.186 g/cm3 (without specifying a composition). Under reflected light the mineral appears brownish gray with very weak bireflectance and weak to moderate anisotropism (gray colors with a cream tint). Internal reflections and pleochroism were not observed. The reflectance values with a 20 nm interval vary from 36.8% (700 nm) to 37.5% (400 nm) for ferhodsite and from 32.8% (700 nm) to 33.8% (400 nm) for the Ir-rich variety described as “Ir ferhodsite.” The values for the COM wavelengths [Rmax/Rmin (nm)] are: 35.6/33.0 (470), 36.0/33.8 (546), 36.2/34.0 (589), 37.1/34.8 (650) for ferhodsite, and 30.0/27.0 (470), 31.3/27.5 (546), 31.3/28.3 (589), 32.0/29.3 (650) for “Ir ferhodsite.” The empirical formulas based on 17 apfu are given in the paper separately for each of 19 analyses. The Fe content varies from 2.82 to 3.99 apfu; Rh 0.18–2.68, Ir 0–2.12, Ni 0.50–1.96, Cu 0–1.64, and S 7.98–8.05 apfu. The powder X‑ray diffraction data of ferhodsite are generally similar to those of pentlandite, however differs by an observed displacement or presence in ferhodsite of some lines and by several doublets. The selected lines of the ferhodsite X‑ray powder diffraction pattern [d Å (I%; hkl)] are (lines absent or significantly differ in position or/and intensity compare to pentlandite are in bold): 5.72 50; 111), 3.01 (70; 311), 2.97 (20; 113), 2.81 (30; 302), 2.74 (203; <10), 2.60 (213; <10), 2.50 (20; 400), 2.23(100; 402), 2.21 (30; 204), 1.933 (60, 105), 1.772 (40; 440), 1.367 (30; 614), 1.167 (40; 803). The data was refined in a tetragonal unit-cell with а = 10.009(5) and с = 9.840(8) Å, V = 985.78 Å3, Z = 4, space group P42/n (or, possibly, P4/nmm). The mineral is named after its two main elements but the I-rich compositions are referred to as “Ir ferhodsite.” The type specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, Russia.

Comments and discussion
The published description contains several discrepancies between the data given in the title, abstract and in the paper itself as well as between the text and d-spaces in Table 3. Cell parameters in the abstract are given to 3 significant figures and to 2 in the text. Totals for analyses #2, 4, 15, and 19 in Table 2 are not correct. Significantly, the authors do not discuss or group the analyses that range widely and do not specify which particular data were used for the calculated density. The average of 15 analyses (14 from Nizhniy Tagil and 1 from Konder) is [wt% (range)]: Ir 14.1 (0–29.6). Rh 22.3 (12.4–31.9), Pt 2.1 (0–14.5), Fe 19.2 (17.3–22.5), Ni 7.7 (2.8–11.2), Cu 5.0 (0.0–9.6), Co 2.6 (0–4.8), S 27.1 (25.9–29.7), total 101.1 for ferhodsite (Rh > Ir pfu) and of 4 analyses of “Ir ferhodsite” (Ir > Rh pfu) from Nizhniy Tagil: Ir 32.7 (26.0–37.9). Rh 7.1 (1.7–12.5), Pt 0.8 (0–2.1), Fe 18.1 (14.8–21.4), Ni 7.5 (4.4–10.7), Cu 8.2 (4.8–10.0), Co 1.7 (0–3.7), S 24.2 (23.4–25.0), total 100.1. The empirical formulae based on 17 apfu for the average compositions are accordingly: (Fe3.35Rh2.11 Ni1.28Cu0.77Ir0.72Co0.43Pt0.11)Σ8.77S8.24 (Dcalc = 6.573 g/cm3) and (Fe3.43Ir1.80 Cu1.36Ni1.35Rh0.73Co0.31Pt0.04)Σ9.02S7.98 (Dcalc = 7.143 g/cm3). Additionally, the deposited type specimen mentioned in the paper (catalogue #93467) considered as holotype is actually the Ir-rich variety but another one (catalogue #93468), considered as co-type (not mentioned in the paper) is an Rh-rich variety (Dmitriy Belakovskiy, pers. comm., 2017). The only reference in the paper is a now dated mineralogy textbook and the only discussion to related minerals is to pentlandite analyses, nor is there any discussion of the pentlandite structure and ionic radii of substituting elements. There is also no mention of a compositionally similar mineral reported from Ethiopia (Cabri et al. 1981) (UM1981- 08-E:FeIrPtRhRu in the IMA list of valid unnamed minerals http:// pubsites.uws.edu.au/ima-cnmnc/Valid2012.pdf) (Smith and Nickel 2007) and another one from the UG-2 chromitite of Bushveld Complex, South Africa (Junge et al. 2014). Another relevant Rh-bearing Ni-Fe sulfide with 24 to 31% Rh reported recently (after the abstracted paper published) from the Coldwell Complex (Ontario) (Good et al. 2017). The names ferhodsite and Ir-ferhodsite are unfortunate since there are clearly compositions with Rh > Ir, Ir > Rh, Ni > Ir or Rh, and Cu > Ir where Fe varies (apfu) from 2.909 to 3.989 for Ir > Rh and from 2.997 to 3.767 for Rh > Ir. There is no evidence that any of these elements occur in specific structural positions, thus there is not enough data to consider whether ferhodsite is actually a single mineral species or several different species. Similarly, one does not know which particular grain was used for reflectance measurements. Criddle and Stanley (1993) include reflectance data for several related minerals, but one is unable to further evaluate the mineral based on the ferhodsite published information.

Lo status della specie, regolarmente approvata, è da considerarsi dubbio.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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