Willhendersonite (KCaAl3Si3O12 · 5H2O) is a new zeolite which is isostructural with chabazite, (Ca,Na2)[Al2Si4O12 · 6H2O. It occurs in the San Venanzo Quarry, Terni, Umbria, Italy in cavities of Quaternary lavas as “trellis-like” twinned aggregates, and as tabular crystals in a limestone xenolith from the Ettringer Bellerberg near Mayen, Eifel, Germany. It is triclinic, space group P1. Single crystals from Mayen have the unit cell parameters a = 9.23(2), b = 9.21(2), c = 9.52(2)Å, α = 92.7(1), β = 92.4(1), γ = 90.1(1), V = 808Å3, Z = 2. Willhendersonite is colorless and transparent, and it has a vitreous luster. The Mohs’ hardness is 3. The measured and observed densities are 2.18 and 2.20 g/cm3, respectively. Willhendersonite is optically biaxial with α = 1.505(3), β = 1.511(3), γ = 1.517(3) and 2V = 87(3)°. The strongest lines in the diffraction pattern are: (d, I/Io) 9.16, 100; 5.18, 30; 4.09, 40; 3.71, 30; 2.907, 60; 2.804, 50.

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