The status of zaratite: investigation of the type specimen from Cape Ortegal, Galicia, Spain

摘要

Three zaratite samples from Cape Ortegal (type locality, Spain), Texas, Lancaster County (Pennsylvania) and Heazlewood (Tasmania) were analyzed by electron-probe micro-analysis (EPMA), environmental scanning electron microscopy with energydispersive spectrometer (ESEM-EDS), X-ray diffraction (XRD), differential thermal analysis and thermogravimetry (DTA-TG), micro-Raman and Fourier-transform infrared spectrometry (FTIR). The empirical formulae calculated from EPMA exhibit different molar ratios (Ni/CO_3, Ni/OH and H_2O/Ni), showing that these three samples referred to as ‘zaratite’ are chemically distinct; they are actually close to either anhydrous zaratite (Ortegal), nullaginite (Tasmania) or gaspéite (Pennsylvania). The XRD patterns of the three samples only validate their low-crystallinity character with variable mineral inclusions. Raman and FTIR spectra confirm large similarities among the three samples, in line with the general chemical kinship of these nickel hydroxycarbonate (± hydrate) materials. Our data suggest that the natural nickel hydroxycarbonate materials, collectively referred to as ‘zaratite’ on the basis of colour, occurrence and poor crystallinity, are in fact so diverse in terms of Ni/C/H ratios that the ‘zaratite’ term should not deserve a species status and so remains highly questionable.