On the new mineral theophrastite, a nickel hydroxide, from Unst, Shetland, Scotland

The new mineral theophrastite, Ni(OH)2 , from Unst, is Mg-bearing and occurs associated with a very poorly crystalline Ni-containing mixed hydroxide of the pyroaurite type and/or zaratite on chromitite. X-ray powder diffraction data show a shift in d spacing toward brucite compared with pure Ni(OH)2. Infrared, thermal, chemical, optical, and physical data are presented together with indexed powder data for fifteen lines ranging from d 4.66A to 0.90A. The solid-solution series brucite-theophrastite is briefly discussed. SEVEN Heddle specimens* of chromitite from Hagdale Quarry, Unst, held in the Scottish Mineral Collection, are partially encrusted with 'emeraldnickel' and zaratite according to Heddle (1901). Macpherson and Livingstone (1974) demonstrated this material to be a mixture of nickel hydroxide, Ni(OH)2, and zaratite. Recent work on this assemblage indicates the nickel hydroxide to be Mg-bearing theophrastite, mixed on a sub-micron scale with a very poorly crystalline Ni-containing mixed-hydroxide of the pyroaurite type and/or zaratite. The natural occurrence of nickel hydroxide was first reported by Williams (1960) although no mineral name was given to the compound. In late 1979 a submission was made by the above authors of this paper to the IMA, endeavouring to establish a mineral name for the compound (Ni,Mg)(OH)2. The material investigated came from Hagdale, Unst, and although associated with a nickel* RSM Nos. 303.2,4,5,6; 338.109 and 493.bl and b2. t Now published: Marcopoulos and Economou (1981). t~ Copyrifht the Mineralogical Society containing mixed-hydroxide or zaratite from which it could not be separated, a detailed investigation fully characterized the mixture. By submitting to the IMA a priority date was established for the Unst material, however, two months after that date another submission was received for pure Ni(OH)z from Greece, by Th. Marcopoulos and M. Economou. In the vote the Unst material and name were narrowly defeated in favour of the Greek Ni(OH)2, which is now known as theophrastite.t Difficulties arose in trying to establish a joint publication with the Greek authors and hence the Unst data are presented separately herewith under the name theophrastite. On the chromitite surfaces two colour variants, though mineralogically similar, are present: the first is a bright emerald-green, gel-like, clear coating whereas the second is pale blue-green and 'opaque'. A single chromitite specimen invested with gel-like and 'opaque' theophrastite, lacks (according to the Museum register entry) detailed locality information and is simply labelled 'zaratite-Canada' (RSM No. 1890-114-1976). This specimen is almost identical to the one from Hagdale Quarry (and is possibly a duplicate) where the gel-like and 'opaque' material collectively cover an area of approximately 60 cm 2. The Hagdale specimens contain minor aragonite, chlorite, and goethite together with scarce citron-yellow reevesite and honessite (Bish and Livingstone, 1981) and very rare pentlandite and heazlewoodite. 2 A. LIVINGSTONE AND D. L. BISH FIG. 1. Scanning electron micrograph showing plates of theophrastite blue-green sample from Hagdale, Unst. Physical and optical properties. Both colour types are devoid of any detectable cleavage or fracture and electron microscopy demonstrates that the Hagdale blue-green type possesses a platy morphology (fig. 1). The lustre is dull for the blue-green variety and resinous for the gel-like material. The Mohs hardness for both varieties is 2 and they possess the same greenish-white streak and specific gravity, which is 3.6. The latter value is considerably lower than that of theophrastite from Vermion, Greece (4.00) or synthetic Ni(OH)2 (3.95) and reflects the considerable amount of mixedhydroxide and/or zaratite (sp. gr. c.2.6) in the material. Optically only one refractive index for each variety could be obtained: n 1.66 (blue-green) and n 1.613 (emerald-green). In transmitted light, grains are obscurely platy to fibrous and nonpleochroic. No fluorescence was observed when examined under longor short-wave ultra-violet light. Using hot, very dilute 5 % v/v HC1 both varieties are rapidly dissolved. X-ray data. As reported by Macpherson and Livingstone (1974) X-ray powder data agree closely with those for synthetic Ni(OH)2. However, if numerous minute fragments are X-rayed individually then appreciable deviation from synthetic Ni(OH)2 d values may be observed, the shift being towards larger values. A solid-solution series would seem to exist between brucite and theophrastite, for both are hexagonal and possess the same space group P3 ml. Natta and Passerini (1928) demonstrated that the synthetic compound (Ni,Mg)(OH)2 50 mol. ~ Ni(OH)2 and Mg(OH)2 had cell parameters intermediate between those of synthetic Ni(OH)2 and Mg(OH)2, and the cell parameters are functions of the changing Ni :Mg ratio throughout the series (G. W. Brindley, pers. comm., 1981). Thus the X-ray powder pattern can be used to estimate composition. A small bulk sample of blue-green Unst theophrastite was prepared and TABLE I. X-ray data for blue-green theophrastite from Haodale, Unst, Shetland I~,. dm,~. dc,l~, hkl 90 4.666 4.666 001 60 2.712 2.712 100 100 2.348 2.345 101 70 1.768 1.768 102 70 1.564 1.565 110 50 1.482 1.484 111 30 1.351 1.349 103 30 1.300 1.300 112 20 1.171 1.172 202 5 1.103 1.103 113 5 1.073 1.073 104 10 1.023 1.022 203 15 0.998 1.001 211 15 0.937 0.938 212 10 0.902 0.904 300 Cu-K~t, 114.6 mm diameter camera, Ni filter, estimated intensities. The data were indexed on a hexagonal cell, a = 3.131, c = 4.666A, V = 39.61A a. X-rayed and d spacings together with cell parameters, are presented in Table I. Tiny single fragments from both the 'Canadian' and Unst samples were also X-rayed and the fragments then analysed under the electron probe microanalyser for Mg and these results are given in Table II. Using the c value of the bulk sample, together with those for pure Ni(OH)2 (4.605A) and brucite (4.769/~) the Unst theophrastite composition could be expressed as (Ni62.aMg37.z)(OH)2. Likewise, from Table II the c parameters indicate a range from (Niaa.4Mgll.6)(OH)2 to (Mg57.9Ni42.1)(OH)2. Considerable discrepancies arise between the calculated MgO contents for the above compositions and the values determined by electron probe microanalysis. These are doubtless due to difficulties encountered in probing very small areas plus sample inhomogeneities. The possibility that the theophrastite is randomly interstratified with a mixed-hydroxide mineral of the pyroaurite type cannot be disTABLE II. Variation of cell parameters with MgO content (determined by EPMA) of four individual theophrastitezaratite fragments