macla nº 15 . septiembre 2011 revista de la sociedad española de mineralogía 197

Talc- and Serpentine-like “Garnierites” from Falcondo Ni- Deposit (Dominican Republic): a HRTEM approach / CRISTINA VILLANOVA-DE-BENAVENT (1,*), FERNANDO NIETO (2), JOAQUÍN A. PROENZA (1), SALVADOR GALÍ (1)

(1) Departament de Cristal·lografia, Mineralogia i Dipòsits . Facultat de Geologia. Universitat de Barcelona. C/ Martí i Franquès s/n. 08028 Barcelona, Catalunya (España) (2) Departamento de Mineralogía y Petrología e Instituto Andaluz de Ciencias de la Tierra. Universidad de Granada-CSIC. Campus de • Fuerteventura s/n. 18002 Granada (España)

INTRODUCTION. from Falcondo Ni-laterite deposit. These “garnierites” mainly occur as mm-cm results are compared to those previously vein fillings in fractures, but also as “Garnierites” represent significant Ni ore obtained through powder XRD and EMP coatings, boxworks and different kinds minerals in the lower horizons of many (CCiT, Universitat de Barcelona). of breccias, within the saprolite horizon Ni-laterite deposits worldwide (e.g. or near the unweathered , Freyssinet et al., 2005). They consist of Preliminary data allows to classify the close to the base of the lateritic profile. a green, fine-grained mixture of hydrous sampled “garnierites” into two groups, Ni-bearing magnesium phyllosilicates, which display two well distinguishable METHODOLOGY. including serpentine, , , greenish colours in hand specimen: smectite and chlorite (e.g. Brindley and A representative sample, containing Hang, 1973; Springer, 1974; Brindley et • Bluish bright-green “garnierites” strongly serpentinized peridotite al., 1979). Thus, “” is a general display colloform textures under the (saprolite) cross-cut by talc-like and descriptive term and is not recognized optical microscope. Their serpentine-like “garnierite” veins, was as a species by the IMA compositions plot within the talc studied through HRTEM. This sample Commission on New Mineral and [Mg3Si4O10(OH)2] (kerolite)-willemseite was prepared in polished thin section Mineral Names (CNMMN). For this [Ni3Si4O10(OH)2] () series, and with Canada balsam. Two areas were reason, “garnierites” have been their diffraction pattern shows basal selected: talc-like and serpentine-like, classified as “serpentine-“, “talc-” and spacings of ~10 Å, so it represents a respectively. These were detached, - “clay-like garnierites”, respectively (e.g. talc-like or a 10 Å–type “garnierite”. thinned by a Gatan 600 ion mill and Brindley and Maksimovic, 1974). were carbon coated. Furthermore, bluish • Yellowish pale-green “garnierites” bright-green and yellowish pale-green Furthermore, hydrated Ni-rich talc develop cryptocrystalline aggregates “garnierites” were separated by hand- minerals have been commonly referred under the optical microscope. These picking, ground in an agate mortar and to as kerolite [Mg3Si4O10(OH)2·(H2O)] and “garnierites” have variable put on a copper grid. HRTEM images, pimelite [Ni3Si4O10(OH)2·(H2O)]. Pimelite compositions near the lizardite electron diffraction patterns and was described as a hydrated Ni-talc (e.g. Mg3Si2O5(OH)4 -népouite microanalysis in STEM mode from the Springer, 1974), but was also Ni3Si2O5(OH)4 join and the XRD two detached specimens and the two determined to belong to the smectite patterns present spacings of ~7 Å, grids were obtained in a Philips CM20, group by some authors (Faust, 1966). corresponding to a serpentine-like or a equipped with an EDX detector, at 200 Although kerolite is not recognized by 7 Å-type “garnierite”. kV (CIC, Universidad de Granada). the CNMMN, and pimelite has been recently discredited (Burke, 2006), these “GARNIERITE” OCCURRENCE IN RESULTS. terms are still used nowadays (Wells et FALCONDO Ni-LATERITE DEPOSIT. al., 2009). HR-TEM imaging. Hydrous Ni- in the Several studies on “garnierites” based Dominican Republic have been The specimen containing talc-like on X-ray diffraction (XRD), Scanning developed during the Miocene on the “garnierites” presents sets of Electron Microscopy (SEM), Electron main ophiolitic peridotite belt in the superimposed bundles and/or plates Microprobe (EMP) themogravimetric and Cordillera Central. This is an elongated with basal spacings near 9.5 Å (Fig. 1a). spectrometric techniques have been body, 4-5 km wide and 95 km long, published. However, High Resolution composed by serpentinized harzburgites, In the specimen with serpentine-like Transmission Electron Microscopy dunites and lherzolites (Lewis et al., “garnierites”, talc bundles coexist with (HRTEM) detailed studies are still scarce 2006; Proenza et al., 2007). two kinds of serpentine microstructures, (Soler et al., 2008). with spacings of about 7 Å. The first type Ni-serpentines and “garnierites” are the develops sectored fibres, with an angle This work presents first HRTEM imaging main Ni-ore-bearing assemblage in the of 24º between adjacent (001) lattice and EDX microanalysis (AEM) results of Falcondo Ni-laterite profile (Lewis et al., planes, so-called polygonal serpentine talc- and serpentine-like “garnierites” 2006). Both talc- and serpentine-like (Fig. 1b). The second type develops

palabras clave: “Garnieritas”, Tipo talco, Tipo serpentina, República key words: “Garnierites”, Talc-like, Serpentine-like, Dominican Dominicana, MET alta resolución. Republic, HRTEM. resumen SEM 2011 * corresponding author: [email protected]

macla nº 15 . septiembre 2011 revista de la sociedad española de mineralogía 198

weakly round cylindrical fibres (Fig. 1c). a.p.f.u.). ACKNOWLEDGEMENTS. Both are hollow-cored or contain talc bundles in the centre. These features As shown in figure 2, AEM results plot This research has been financially were described and identified as within or near the compositional field as supported by the Spanish project lizardite and , respectively, by those previously obtained through EMPA CGL2009-10924 and a PhD grant to Baronnet & Devouard (2005) and in similar samples. Moreover, serpentine CVdB from the Ministerio de Educación references therein. analyses are deviated towards the talc (Spain). Technical support by M.M. Abad (kerolite)-willemseite (pimelite) series, and I. Nieto (Universidad de Granada) due to the coexistence of serpentines was essential to this study. JAP with talc-like “garnierites” in the same gratefully acknowledges the help and area. hospitality extended by the staff at Falcondo Xtrata mine.

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