Schindler et al. Geochem Trans (2019) 20:1 https://doi.org/10.1186/s12932-019-0061-3 Geochemical Transactions RESEARCH ARTICLE Open Access The formation of spinel-group minerals in contaminated soils: the sequestration of metal(loid)s by unexpected incidental nanoparticles Michael Schindler1* , Haley Mantha2 and Michael F. Hochella Jr.3,4 Abstract Mineralogical studies of contaminated soils afected by smelter emission and dust from mining activities indicate that minerals of the spinel group are one of the common hosts of metal-bearing contaminants. Spinel group minerals typically originate from high temperature processes, but an increasing number of studies indicate that metal-bearing spinel group minerals can also form under ambient Earth surface conditions in surfcial soils. In this contribution to honor Donald Sparks, we show that the spinels Zn-bearing magnetite (Zn 0.5Fe2.5O4) and minium (Pb3O4) form during low temperature alteration of Pb-bearing silica glass in surfcial organic rich soils in proximity to a former Cu-smelter in Timmins, Ontario, Canada. The glass most likely formed during high-temperature processes and has been either emitted by the smelter or wind-blown from waste rock piles to near-by soils. The alteration of the glass by percolating pore solutions has resulted in the formation of large micrometer-size dendritic etch features and in nanometer-size dendritic alteration halos composed of nano-size prismatic crystals of Zn-rich magnetite and spherical nanoparticles of minium. Both spinel-type phases are embedded in an amorphous silica matrix which formed during the alteration of the glass at low temperature. A review on the occurrence of spinel-group minerals in smelter-afected soils or mine tailings indicates that the formation of these minerals under ambient Earth surface conditions is quite common and often results in the sequestration of contaminants such as Cu, Ni, Zn and Sb. The pedogenic spinels often occur as euhedral crystals in nano-size mineral assemblages within alteration features such as dendritic etch patterns, mineral surface coatings and mineralized organic matter. Their well-developed crystal forms indicate that (a) they have not formed during a rapid cooling process in a smelter or refnery which typically creates spherical particulate matter, and (b) they have not been part of particulate matter added via fuvial or Aeolian processes which most commonly yield anhedral morphologies. The formation of nano-size spinel-group minerals in low temperature environmental settings may lead to the long-term storage of metal(loid)s in mineral phases and their transport over vast distances via fuvial, alluvial and Aeolian processes. Keywords: Spinels, Contaminated soils, Pedogenic, Mineral nanoparticles, Mineral surface coatings, Organic matter, Low temperature *Correspondence: [email protected] 1 Department of Earth Sciences, Laurentian University, Sudbury, ON, Canada Full list of author information is available at the end of the article © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/ publi cdoma in/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Schindler et al. Geochem Trans (2019) 20:1 Page 2 of 12 Introduction spinels such as magnetite Fe3O4, maghemite, Fe2O3 and Incidental nanoparticles (nanomaterials unintention- titanomagnetite, FeTiO4, are common and occur in igne- ally produced as a result of any form of direct or indirect ous massifs as cumulate strata, in metamorphic terrain human infuence or anthropogenic process) are com- and in sedimentary deposits either as refractory minerals mon in the most afected areas of Earth, including in and or in association with lateritic-type deposits. around manufacturing facilities, mining areas, power Te upper horizons of many modern and buried soils plants, waste water treatment plants, agricultural lands, have higher concentrations of ferrimagnetic minerals and surface and subsurface waters associated with all of such as magnetite and maghemite than the parent mate- these areas [1, 2]. Knowledge about the formation and rial from which they were originally derived [10]. Litho- chemical and physical interactions of incidental nano- genic magnetite is a common mineral in these soils as particles with their surroundings is very often necessary part of the coarse, heavy mineral fraction. Te occur- to understand the fate of pollutants in the environment. rence of pedogenic magnetite has only been established Tese nanoparticles are known to either structurally since the late 80’s as researchers showed that magnetite incorporate metal(loid) contaminants from their imme- can form through abiotic [7] and biotic [11] processes diate environment via adsorption or absorption pro- in soils. Pedogenic magnetite occurs commonly in the cesses, and potentially transport them over vast distances nano-size fractions of soils and is a common reduction via fuvial, alluvial and Aeolian processes [3, 4]. product of ferric oxyhydroxides in microaerobic and Mineralogical studies of soils afected by emissions anaerobic sediments and soils [9, 10, 12]. Te biotic for- from smelters and refneries or by dust windblown from mation of spinels typically involves biomineralisation by nearby or far-away mining activities indicate that miner- Fe-reducing bacteria which gain energy through oxida- als of the spinel group are one of the major hosts of metal tion of organic matter using nitrate and Fe(III). Tese contaminants [5, 6]. We suspect that mineralogical stud- bacteria can be magnetotactic such as Aquaspirillum ies of afected soils have overlooked the sequestration magnetotactum or reducing such as Geobacter sulfurre- of contaminants by pedogenic minerals of the spinel- ducens and Shewanella oneidensis [9, 11]. group, such as magnetite, because they were thought not Abiotic formed magnetite NPs can form by co-pre- to form under ambient Earth surface conditions [7]. In cipitation reactions that, as shown in analogous synthe- addition, fnding and studying minerals in the nano-scale sis experiments, follow a number of diferent pathways. range is expensive and difcult. In this study to honor the However, the predominant process is a topotactic trans- achievements of Donald Sparks, we will show that euhe- formation of goethite to magnetite [12]. Nucleation dral prismatic crystals of Zn-rich magnetite, and nano- and growth of abiotic magnetite NPs can also precede particles of the mineral minium (Pb 3O4), both from the through rapid agglomeration of spheroidal Fe-hydroxide spinel group, form during low-T alteration of Pb-bearing nanoparticles particles (5–15 nm in diameter) without silica glass in the contaminated soils around the former the involvement of an amorphous bulk precursor phase Kidd Creek smelter complex in Timmins, Ontario, Can- [13]. In aerated environments magnetite is unstable and ada. Te occurrence of these pedogenic spinel phases will is subject to weathering. Maghemite is the main weath- be compared with previous observations on the forma- ering product of magnetite. Maghemite nanoparticles in tion of spinel group minerals during low-T abiotic and soils can also form through dehydration of lepidocrocite biotic-controlled alteration processes in mineral surface nanoparticles [12]. coatings and mineralized organic matter. Methodology Background information on spinel‑group minerals Background information on the Kidd Creek metallurgical Minerals and phases of the spinel group are of great envi- site ronmental, geological and industrial importance as they Te Kidd Creek metallurgical site, located within the can incorporate a large variety of di-, tri-, tetra- and pen- city limits of Timmins, Canada (Fig. 1a), was in opera- tavalent cations [8]. Te general formula of simple spinels tion for 30 years, closing in 2010. Te ore processed at is AB2O4 where the divalent A cation can either occupy a this location contained predominantly chalcopyrite tetrahedral- (“normal” spinel) or octahedral-coordinated (CuFeS2), pyrite (FeS2), bornite (Cu5FeS4), pyrrhotite site (reverse spinel) in the spinel structure. (Fe S 0–0.2), sphalerite (ZnS), and galena (PbS) (1−x) x = Many spinel group minerals have limited geologi- [14]. Te ore was shipped via train from the mine site to cal occurrences, but spinel-group minerals containing the metallurgical site for processing (27 km away). Te Zn, Cr, Fe and Ti are abundant enough to be considered Mitsubishi copper smelting process was employed at important ore minerals. Tese spinel ore minerals include start-up with furnaces operating around 1200–1300 °C willemite, Zn2SiO4 and chromite, FeCr2O4. Iron-rich [15]. From 2002 to 2009, the average amount of total Schindler et al. Geochem Trans (2019) 20:1 Page 3 of 12 Fig. 1 a Map of Canada indicating the location of Timmins, Ontario with a red point symbol; b photographic image of the sampling location, circa 1.3 km from the former Kidd Creek smelter complex particulate matter (PM < 100 μm) released to the atmos- Measurements were taken with an accelerating voltage phere was 844 t/year [16]. of 200 kV and