1. Introduction Huntite [Mg 3 Ca(CO 3 ) 4 ] and hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 .4H 2 O] are classified as metastable carbonate min- erals (Kinsmann 1967, sánchez-Román et al. 2011).The formation of huntite has been attributed to several mecha- nisms and environments some of which are listed below:  Precipitation from percolating waters moving through magnesium-rich rocks such as magnesite, dolomite and/or hydromagnesite deposits (e.g. Faust 1953, sKinneR 1958, tRailKill 1965, zachmann 1989).  Bacterial activities during the initial formation of sab- kha (PeRthuisot et al. 1990).  Precipitation from Mg-bearing pore waters during early diagenesis (Kinsman 1967), at the expense of do- lomite which becomes unstable when the Mg/Ca ratio is higher than required for dolomitization (e.g. iRion & mülleR 1968, mülleR et al. 1972).  Near-surface weathering of serpentinite and highly serpentinized rocks (e.g. DamoDaRan & somaseKaR 1975, nemec 1981, stangeR & neal 1994, BashiR et al. 2009, eslamizaDeh et al. 2014, losos et al. 2013). Although huntite is a rare carbonate mineral, it has been found in a wide range of geological settings, includ- ing weathered volcanic tuff sequences, coastal sabkhas, karstic terrains, continental lacustrine environments, highly alkaline carbonate playas and weathered serpenti- nized rocks (F aust 1953, cole & lanchucKi 1975, calvo et al. 1995, zeDeF et al. 2000, BashiR et al. 2009). Similarly, hydromagnesite and magnesite occur in a variety of geological settings and environments. Several interactive mechanisms such as supergene, hypogene, and combined supergene-hypogene processes have been proposed to explain the origin of magnesite in ultramafic settings (zachmann 1989, stamataKis 1995, Russell et al. 1999, F RanK & F ielDing 2003). Ultramafic rocks are con- sidered to be the source of the Mg for the fluid (o’neil & BaRnes 1971, zeDeF et al. 2000, miRnejaD et al. 2008). In these settings, several carbonate sources can be distin- On the occurrence of Mg- and Fe-rich carbonate mineral assemblages hosted in the Nain ophiolite mélange, Central Iran and their industrial potential Alireza Eslami, Michael G. Stamatakis, Maria Perraki, Charalampos Vasilatos and Luke Hollingbery With 5 figures and 2 tables Abstract: In the Nain ophiolite mélange, central Iran, off-white mineral assemblages occur as nodular magnesium rich carbon- ates and thin veinlets disseminated within an earthy serpentinite groundmass. They are related to tectonically disturbed, strongly weathered zones of the ultramafic rocks. Combined XRD, SEM and TG/DTA analysis revealed that the mineralogy of the Mg-rich carbonate is varied. Ten distinct paragenetic assemblages containing hydromagnesite, pyroaurite, manasseite, brugnatellite, hydro- talcite, aragonite, and/or huntite were found. The mineral assemblages formed as the result of precipitation from percolating Mg- rich meteoric waters through brecciated serpentinites. The source of Mg in excess in the groundwater is attributed to the hydrolysis of Mg-rich minerals in the predominant serpentinized ultramafic rocks. Selected hydromagnesite-rich samples were tested as fire retardants. Even though hydromagnesite is the predominant mineral phase, the economic importance of the mineral assemblages in total is limited mainly because of the insufficient whiteness and the presence of Fe-rich minerals that cause undesirable thermal reactions. Key words: nodular Mg-rich carbonates, serpentinite, hydromagnesite, huntite, pyroaurite, fire retardants, economic importance, Nain ophiolite mélange, Iran © 2014 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany www.schweizerbart.de DOI: 10.1127/njma/2014/0271 0077-7757/2014/0271 $ 3.25 N. Jb. Miner. Abh. (J. Min. Geochem.) 192/1 (2015), 59–71 Article Published online October 2014; published in print January 2015 E F-0271_njma_192_1_0059_0071_Eslami_0271.indd 59 11.11.2014 15:01:49 eschweizerbart_xxx