Résumé
In the Nain ophiolite melange, central Iran, off-white mineral assemblages occur as nodular magnesium rich carbonates 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, hydrotalcite, 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.