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Garutiite, (Ni,Fe,Ir), a new hexagonal polymorph of native Ni from Loma Peguera, Dominican Republic
Authors: ANDREW M. MCDONALD, JOAQUIN A. PROENZA, FEDERICA ZACCARINI, NIKOLAY S. RUDASHEVSKY, LOUIS J. CABRI, CHRIS J. STANLEY, VLADIMIR N. RUDASHEVSKY, JOAN C. MELGAREJO, JOHN F. LEWIS, FRANCISCO LONGO and RONALD J. BAKKER
ABSTRACT
Garutiite (Ni,Fe,Ir) is a new hexagonal polymorph of native Ni discovered in chromitite from Loma Peguera, Dominican Republic. The mineral was identified in heavy mineral concentrates obtained through the use of electric pulse disaggregation (EPD) and hydroseparation (HS) techniques. It forms as anhedral, botryoidal grains typically 10–60 µm in size (maximum of 110 µm). Grains are single or composite, frequently porous and zoned, and occasionally display an unusual lamellar internal texture. Associated minerals include hexaferrum, ferrian chromite, chlorite-group minerals, serpentine-group minerals, awaruite, irarsite, laurite, native Ru and unidentified species including Ru–Os–Ir-Fe and Pt–Ni–Fe–Ir compounds, Pt(Ni,Fe)3, (Fe,Ru,Ni,Os,Ir,Co)2S and RhNiAs. The mineral is megascopically grey to grey-black with a metallic luster. In plane-polarized light, garutiite is white in color, exhibits a very weak anisotropy, and no pleochroism, bireflectance or internal reflections were observed. No cleavage was noted and the hardness could not be determined owing to the porous nature of the mineral. The calculated density is 11.33 (1) g/cm3. Reflectance values (%) in air are: 63.8 at 470, 65.9 at 546, 67.0 at 589 and 68.0 at 650 nm. The average result of electron microprobe analyses (n = 42 from 27 grains) is: Ni 27.91, Fe 19.94, Ir 43.78, Pt 6.98, Co 0.55, Cu 0.43, Ru, 0.50, Rh 0.74, Os 0.67, total 101.51 wt%, corresponding to (Ni0.421Fe0.316Ir0.202Pt0.032Co0.008Cu0.006Rh0.006Ru0.004Os0.003)Σ1 or the simplified formula, (Ni,Fe,Ir). Garutiite is the Ni analogue of hexaferrum, osmium and ruthenium and is classified as belonging to the osmium group. As such, the mineral is considered to be hexagonal, crystallizing in space group P63/mmc with a 2.6941(4) and c 4.2731(6) Å, V = 26.86(1) Å3, Z = 2. The strongest lines of the X-ray powder diffraction pattern [d(in Å)(I)(hkl)] are: 2.330(50)(100), 2.136(30)(002), 2.046(100)(101), 1.576(30)(102), 1.3470(40)(110), 1.2155(40)(103). Based on its morphology, internal texture, and the associated minerals, garutiite is interpreted to be secondary in origin, i.e., having formed at low temperatures during post magmatic processes, such as serpentinization and/or lateritization. The name honors Prof. Giorgio Garuti, in recognition of his contributions to the understanding of the mineralogy of platinum-group elements.
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