"Rock of the Month # 232, posted for October 2020" ---

Gaspeite, (Ni,Mg,Fe^II)CO₃, is a trigonal carbonate of the calcite group, forming a series with magnesite. It may occur massive, or forming crystals to 1 mm in size. The type locality is a vein 50 cm wide in silicified dolomite host rock (Kohls and Rodda, 1966), at Lemieux township in the Gaspe peninsula of Quebec, whence the name (see also Horvath, 2003). At the type locality, the green carbonate is associated with small amounts of other Ni minerals, namely millerite, niccolite, annabergite and gersdorffite. Magnesite, dolomite and minor serpentine are also present. Analysis of a gaspeite mineral separate revealed a magnesian gaspeite composition, containing 35 wt.% NiO, 17.3 wt.% MgO and 5.7 wt.% FeO (Kohls and Rodda, 1966). The mineral is quite distinctive in thin section (Figs. 2,3).

Fig. 2a,b: Two photomicrographs of equant gaspeite crystals in much coarser clear calcite, which shows typical rhombohedral cleavage planes. Photos in plane and (right) crossed-polarized transmitted light, 50X, long-axis field of view 1.7 mm. The gaspeite has much lower birefringence than the coarse host calcite. Polished thin section prepared by Precision Petrographics Ltd.

Gaspeite is found also at the Otway mine and Widgiemooltha in Western Australia; Richelsdorf, Germany; Laurion, Greece, and in Bosnia and Herzegovina (Bernard and Hyrsl, 2015, p.273). The 132 North deposit at Widgiemooltha is a small komatiite-hosted Ni sulphide orebody that commenced open-pit mine operation in 1990. It is notable for having the greatest diversity of Ni secondary minerals anywhere in the world (Nickel et al., 1994; Moore, 2016, p.690). Gaspeite is locally abundant there, forming, in one section of the deposit, veins up to 50 cm thick and 10 m long. Vuggy openings are often lined by tiny gaspeite crystals.

As with primary Ni minerals (which are dominated by sulphides in most deposits), so gaspeite is generally found in association with mafic-ultramafic rocks, as at Dubostica in Bosnia and Herzegovina (Bermanec et al., 2000). At the Otter Shoot in Kambalda, supergene alteration completely converted pyrrhotite and pentlandite into violarite and pyrite, while decomposition of sulphides at the water table produced gaspeite and reevesite, the latter an hydrated Ni-Fe carbonate of the hydrotalcite group (Keele and Nickel, 1974).

Gaspeite is also a secondary mineral at the tiny, highly unusual Bon Accord Ni deposit in the Barberton Mountain Land of northeastern South Africa (Tredoux et al., 1986, 1989). The major phase is trevorite, a Ni spinel, hosted in a metadunite of the Archean Jamestown ophiolite complex (age dated at circa 3500 Ma). The deposit was essentially room- or truck-sized, just 6x3x0.3 metres, grading a remarkable 38 wt.% Ni. This unique deposit was discovered on a hillside in 1920. It may represent a remnant of deep mantle rock, depleted in Cu and S.

Gaspeite is a rather rare mineral, but well characterized (see, e.g., Melgarejo and Martin, 2011; and the RRUFF database ). Beautifully green, fine-grained and taking a good polish, it is favoured as an attractive and unusual stone for lapidary purposes (e.g., Thomas, 2008; Thompson, 2010). The Gaspe rock has some potential in this regard, as noted long since by local prospectors (Gasse et al., 1991). Tens of tonnes of similar rock has been sold as lapidary and jewellery rough from nickel operations In Western Australia (Thompson, 2010).

Fig. 3a,b,c: Three more transmitted-light photomicrographs of equant gaspeite crystals in the coarse calcite host. Photos in plane- (top) and crossed-polarized light (centre). At base, a sensitive tint plate has been added to highlight the sector zoning of the gaspeite crystallites. All at 50X magnification, long-axis field of view 1.7 mm. Microscopes, cameras and technical support by Opti-Tech Scientific of Whitby, Ontario .

References

Bermanec,V, Sijaric,G, Kniewald,G and Mandarino,JA (2000) Gaspeite and associated Ni rich minerals from veins in altered ultrabasic rocks from Dubostica, Bosnia and Herzegovina. Can.Mineral. 38, 1371-1376.

Bernard,JH and Hyrsl,J (2015) Minerals and their Localities. Granite, Prague, Czech Republic / Mineralogical Record Bookstore, Tucson, 3rd edition, 920pp.

Gasse,G et al. (1991) Mineral Potential of the Bas Saint Laurent and the Gaspésie. Association des Prospecteurs Gaspésiens, Sainte Anne des Monts, Quebec, 54pp.

Horvath,L (2003) Mineral Species Discovered in Canada and Species Named After Canadians. Can.Mineral. Spec.Publ. 6, 374pp.

Keele,RA and Nickel,EH (1974) The geology of a primary millerite bearing sulfide assemblage and supergene alteration at the Otter Shoot, Kambalda, Western Australia. Econ.Geol. 69, 1102- 1117.

Kohls,DW and Rodda,JL (1966) Gaspeite, (Ni, Mg, Fe) (CO₃), a new carbonate from the Gaspe peninsula, Quebec. Amer.Mineral. 51, 677-684.

Melgarejo,JC and Martin,RF (2011) Atlas of Non Silicate Minerals in Thin Section. Canadian Mineralogist Spec.Publ. 7, 522pp. plus CD ROM.

Moore,TP (2016) Moore's Compendium of Mineral Discoveries, 1960-2015. Mineralogical Record, Inc., Tucson, 2 volumes, 809+813pp.

Nickel,EH, Clout,JFM and Gartrell,BJ (1994) Secondary nickel minerals from Widgiemooltha, Western Australia. Mineral.Record 25, 283-291,302.

Thomas,A (2008) Gemstones: Properties, Identification and Use. New Holland Publishers (UK) Ltd, 256pp.

Thompson,SE (2010) Gaspeite. Lapidary Journal Jewelry Artist 64 no.1, 16.

Tredoux,M, de Wit,MJ, Hart,RJ, Armstrong,RA, Lindsay,NM and Sellschop,JPF (1989) Platinum group elements in a 3.5 G.a. nickel iron occurrence: possible evidence of a deep mantle origin. J.Geophys.Res. 94B, 795- 813.

Tredoux,M, De Wit,MJ, Hart,RJ, Lindsay,NM and Sellschop,JPF (1986) The Bon Accord trevorite body - a sample of a deep mantle siderophile segregation? Geocongress '86, Extended Abstracts Vol., Jo'burg, 1056pp., 391- 396.