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Kernowite, Cu2fe(Aso4)(OH)4⋅4H2O, the Fe -Analogue of Liroconite from Cornwall, UK

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Kernowite, Cu2fe(Aso4)(OH)4⋅4H2O, the Fe -Analogue of Liroconite from Cornwall, UK Mineralogical Magazine (2021), 85, 283–290 doi:10.1180/mgm.2021.40 Article 3+ Kernowite, Cu2Fe(AsO4)(OH)4⋅4H2O, the Fe -analogue of liroconite from Cornwall, UK Michael S. Rumsey1* , Mark D. Welch1, John Spratt2, Annette K. Kleppe3 and Martin Števko4,5 1Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK; 2Core Research Laboratories, Natural History Museum, London SW7 5BD, UK; 3Diamond Lightsource UK, Harwell Science Park, Chilton, Oxfordshire OX11 0DE, UK; 4Earth Science Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovak Republic; and 5Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, 193 00 Praha, Horní Počernice, Czech Republic Abstract ⋅ The occurrence, chemical composition and structural characterisation of the new mineral kernowite, ideally Cu2Fe(AsO4)(OH)4 4H2O, 3+ ⋅ the Fe -analogue of liroconite, Cu2Al(AsO4)(OH)4 4H2O, are described. Kernowite (IMA2020-053) occurs on specimens probably sourced from the Wheal Gorland mine, St Day, Cornwall, UK, in the cavities of a quartz-gossan rich in undifferentiated micro-crystalline grey sulfides and poorly crystalline arsenic phases including both pharmacosiderite and olivenite-group minerals. The average compos- ition of kernowite determined from several holotype fragments by electron microprobe analysis is Cu1.88(Fe0.79Al0.09)Σ0.88(As1.12O4) ⋅ (OH)4 3.65H2O. The structure of kernowite has been determined in monoclinic space group I2/a (a non-standard setting of C2/c) by single-crystal X-ray diffraction (SCXRD) to R1 = 0.025, wR2 = 0.051 and Goodness-of-fit = 1.112. Unit-cell parameters from SCXRD are a = 12.9243(4) Å, b=7.5401(3) Å, c=10.0271(3) Å, β = 91.267(3)°, V=976.91(6) Å3 and Z = 4. The chemical formula of 3+ ⋅ this crystal indicated by SCXRD from refined site-scattering is Cu2(Fe0.84(1)Al0.16)AsO4(OH)4 4H2O. The network of hydrogen bonding has been determined and is similar to that reported for liroconite from Wheal Gorland by Plumhoff et al. (2020). Keywords: kernowite, liroconite, Cornwall, iron aluminium arsenate, mineral collections, mineral museum, United Kingdom, new mineral (Received 14 April 2021; accepted 6 May 2021; Accepted Manuscript published online: 12 May 2021; Associate Editor: Juraj Majzlan) History and occurrence analogue of liroconite, but the work was never formally written up. Kernowite was identified on an old museum specimen labelled as Due to the historic nature of the specimen, probably collected a liroconite from Wheal Gorland in the Sir Arthur Russell between 215–225 years ago, there is an element of uncertainty Collection of British Minerals housed at the Natural History with respect to its locational provenance, exacerbated in this Museum in London (Fig. 1). The specimen, registered as BM case as there has been informal debate for years questioning if 1964,R8908 was, prior to the ownership of the NHM and all old liroconite specimens are from the commonly attributed Russell before that, in the collection of the important Cornish Wheal Gorland. For lack of a means of verifying the exact source mineral collector and Member of Parliament, Philip Rashleigh location, the attributed locality of Wheal Gorland, given either by (1729–1811) and bears some notable resemblance to one of the Sir Arthur Russell or Philip Rashleigh is, henceforth, assumed to earliest figured examples of liroconite by James Sowerby in be correct. 1803 (Sowerby, 1804). Wheal Gorland was situated in the Parish of St. Day, Cornwall, The specimen was selected as being of interest during a sys- United Kingdom (50°14’30”N, 5°10’58”W) and was first recorded tematic study of liroconite by one of us (MSR) in the mid as a working mine in 1792 (Anon., 1799). It’s history since this 2000’s. Since its discovery, probably in the late 1790’s time is well documented, as is its fame within the mineral collect- (De Bournon, 1801) and approximate naming (as liriconite) by ing community for its rich, beautiful and largely unique suite of Jameson (1821) it has been regularly recognised as varying in col- copper arsenate minerals, including the iconic liroconite. The our from sky-blue to ‘verdigris’ or grass-green, speculation sug- locality and most of its mineral dumps no longer exist, having gested this was due to variation in phosphate–arsenate content been removed or levelled for recent housing developments. (Berry, 1938), but no modern confirmatory work had been per- Only a small dump, preserved as an SSSI (Site of Special formed. Specimen BM 1964,R8908 was selected for study due Scientific Interest) remains. to its unusual dark-green colour, more akin to that of an emerald Kernowite occurs with poorly-crystalline arsenic phases (Fig. 1). Chemical (energy-dispersive spectroscopy) analysis of a including pharmacosiderite and olivenite-group minerals in cav- small fragment suggested that the material was an Fe-dominant ities in a quartz-gossan, rich in undifferentiated micro-crystalline grey sulfides. Recent work by Plumhoff et al. (2020) suggests that the crystallisation of liroconite is related closely to the formation *Author for correspondence: Michael S. Rumsey, Email: [email protected] of arsenic-rich gels/mineraloids and it is suggested here, on the Cite this article: Rumsey M.S., Welch M.D., Spratt J., Kleppe A.K. and Števko M. (2021) ⋅ 3+ basis of the assemblage present, that kernowite is similar. It is Kernowite, Cu2Fe(AsO4)(OH)4 4H2O, the Fe -analogue of liroconite from Cornwall, UK. Mineralogical Magazine 85, 283–290. https://doi.org/10.1180/mgm.2021.40 interesting to note that the mineral and textural associations on © The Author(s), 2021. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland 284 Michael S. Rumsey et al. Table 1. Chemical data (n = 9) for kernowite. Constituent Wt.% Range S.D. Standards Fe2O3 15.38 14.53–15.97 0.51 Hematite CuO 36.00 35.36–37.22 0.63 Cu2O As2O5 31.12 28.25–33.30 0.26 NiAs (syn.) Al2O3 1.15 0.76–1.85 0.31 Corundum (syn.) CaO 0.02 bd–0.06 0.02 Wollastonite SiO2 0.09 0.04–0.14 0.03 Wollastonite H2Ocalc 24.69 24.52–24.92 0.82 Total 104.55 103.73–105.53 N/A S.D. – Standard deviation; bd – below detection. Gorland, providing context to the rarity of the kernowite–liroconite distinction. In a full study of over 60 samples, 25 of which were analysed and ten of which were ‘green’, only one with crystals exhibiting a 0.5 mm green rim and blue cores could be described as kernowite-containing and this was only in the green rims. This colour zonation from a blue core to green rim has been noted in liroconite specimens since at least 1803 (Sowerby) and is now deserving of more attention. Considering the many liroconite samples in existence, it is likely that other specimens bearing ker- nowite are present in public and private collections. However, we stress that accurate chemical analysis is essential for determin- ation, as very few of the ‘green-liroconites’ analysed thus far have been kernowite. Kernowite, is named after the word for Cornwall in the Cornish language, which is commonly spelt ‘Kernow’. As the exact locality attribution of Wheal Gorland, although likely, is Fig. 1. Holotype sample BM 1964,R8908, the large central crystal is ∼12 mm in size, not certain it is reasonable to name the mineral after the region all material used in the study was removed from the remnants of the cavity infill, that we know the sample is from. Furthermore, as the analogue probably the base of a broken crystal present on the left of the image. The large crys- of liroconite, kernowite is a doubly appropriate name, reflecting tals were not sampled so that their form on this historic specimen was preserved. liroconite’s place as one of Cornwall’s most iconic mineral species famous around the world for its striking colour, historic origin this sample are subtly different to the majority of specimens and rarity. Kernowite will join both cornwallite and cornubite labelled liroconite from Wheal Gorland. as markers of the historic importance of Cornwall in mineral An extensive study by one of us (MS) recently identified a sciences. The region recognised by UNESCO as a world heritage second kernowite-containing specimen, also attributed to Wheal site based on its importance to the history of mining. Fig. 2. Plot of the Al–Fe composition of 26 liroconite– kernowite specimens, the most Fe rich being the ker- nowite holotype. The plot additionally features an approximation of how the colour of the sample changes with increasing Fe content from blue through blue–green to green and dark green, many green and blue–green samples still fall in the compositional field of liroconite. Mineralogical Magazine 285 The mineral and name have been approved by the Table 2. Summary of information relating to data collection and structure International Mineralogical Association (IMA) Commission on refinement of kernowite. New Minerals, Nomenclature and Classification (IMA2020-053, Crystal data Rumsey et al., 2020). Holotype material is stored at the Natural Ideal chemical formula Cu2Fe(AsO4)(OH)4⋅4H2O ⋅ History Museum in London, within the Sir Arthur Russell Structural formula (SREF) Cu2Fe0.84Al0.16(AsO4)(OH)4 4H2O Collection of British Minerals, the sample consists of a specimen, Space group* I2/a a (Å) 12.9243(4) two polished microprobe blocks, a number of mounted crystal b (Å) 7.5401(3) fragments for single crystal studies and a small number of loose c (Å) 10.0271(3) tiny fragments, are all registered collectively under the number β (°) 91.267(3) BM1964,R8908.
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