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 CO₃^2--depleted and P- and H-enriched samples from Sacrofano paleovolcano, Lazio, Italy, are studied. Both minerals are monoclinic; latiumite P2₁, a = 12.0206 (3), b = 5.09502 (10), c = 10.8527 (3) Å, β = 107.010 (3)°, V = 635.60 (3) ų and tuscanite P2₁/a, a = 23.9846 (9), b = 5.09694 (15), c = 10.8504 (4) Å, β = 107.032 (4)°, V = 1268.26 (8) ų. The obtained crystal chemical formulae (Z = 2 for both minerals) are [(H₃O)_0.48(H₂O)_0.24K_0.28](Ca_2.48K_0.21Na_0.21Sr_0.06Mg_0.04)(Si_2.86Al_2.14O₁₁)[(SO₄)_0.70(PO₄)_0.20](CO₃)_0.10 for latiumite and [(H₃O)_0.96(H₂O)_0.58K_0.46](Ca_4.94K_0.44Na_0.45Sr_0.09Mg_0.08)(Si_5.80Al_4.20O₂₂)[(SO₄)_1.53(PO₄)_0.33](CO₃)_0.14 for tuscanite. These minerals are dimorphous. Both latiumite and tuscanite show distinct affinity for the PO₄^3- 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.