New data on the crystal chemistry of the natural two-layer aluminosilicates latiumite and tuscanite.

The crystal chemistry of the natural microporous two-layer aluminosilicates (2D zeolites) latiumite and tuscanite is re-investigated based on new data on the chemical composition, crystal structures, and infrared and Raman spectra. The CO 3 2- -depleted and P- and H-enriched samples from Sacrofano paleovolcano, Lazio, Italy, are studied. Both minerals are monoclinic; latiumite P2 1 , a = 12.0206 (3), b = 5.09502 (10), c = 10.8527 (3) Å, β = 107.010 (3)°, V = 635.60 (3) Å 3 and tuscanite P2 1 /a, a = 23.9846 (9), b = 5.09694 (15), c = 10.8504 (4) Å, β = 107.032 (4)°, V = 1268.26 (8) Å 3 . The obtained crystal chemical formulae (Z = 2 for both minerals) are [(H 3 O) 0.48 (H 2 O) 0.24 K 0.28 ](Ca 2.48 K 0.21 Na 0.21 Sr 0.06 Mg 0.04 )(Si 2.86 Al 2.14 O 11 )[(SO 4 ) 0.70 (PO 4 ) 0.20 ](CO 3 ) 0.10 for latiumite and [(H 3 O) 0.96 (H 2 O) 0.58 K 0.46 ](Ca 4.94 K 0.44 Na 0.45 Sr 0.09 Mg 0.08 )(Si 5.80 Al 4.20 O 22 )[(SO 4 ) 1.53 (PO 4 ) 0.33 ](CO 3 ) 0.14 for tuscanite. These minerals are dimorphous. Both latiumite and tuscanite show distinct affinity for the PO 4 3- anion. Hydrolytic alteration of these minerals results in partial leaching of potassium accompanied by protonation and hydration which is an important precondition for the existence of ion/proton conductivity of related materials.