This paper analyses the mineralogical composition, texture, and structure of a stalactite sampled from the city-wall storerooms of the Nueva Tabarca fortress (southeast Spain). This speleothem presents an uncommon mineral assemblage: aragonite, brucite, gypsum, silica, and halite. Internally, it shows complex structure: (1) a central soda-straw composed by aragonite; (2) an external puff-pastry cone-crust formed preferentially by aragonite and brucite; and (3) an internal branching of coralloids, showing a subtle layering between brucite and aragonite. Gypsum, halite, and silica locate in the outer coating of the cone-crust. The sequent mineral precipitation sequence has been established: aragonite > brucite > gypsum/silica > halite. Speleothem formation is directly related to the chemical weathering of the rocks and mortars used as building materials of the city-wall. Brucite precipitates has been always linked to the presence of MgO-based geomaterials. However, the lack of these compounds as building materials in Nueva Tabarca fortress makes this investigation a unique example of brucite precipitation. PHREEQC calculations showed that interaction between pore waters and the minerals of mortar aggregates (dolomite, pyroxene, and amphibole) leads to rich-magnesium solutions. Evaporation modelling of lixiviated waters describes the precipitation of the mineral assemblage of the brucite-aragonite speleothems.
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