Chiavennite occurs as euhedral orange grains and as crusts coating beryl in the alpine pegmatites crossing the Chiavenna ultramafics in the Rhetic Alps of Italy. It is usually associated with bavenite and several other pegmatitic minerals. Chiavennite is optically biaxial, α =1.581(1) and γ = 1.600(1), faintly pleochroic from colorless or yellow (X) to yellow-orange (Z). The empirical formula derived from EDS microprobe analysis and separate analyses for BeO (optical spectrography) and H2O (TGA) is (Ca0.97Na0.05)Σ1.02 Mn0.97(Be1.98Al0.03)Σ2.01(Si4.65Al0.35)Σ5.00O12.63(OH)2.37·2.16H2O or ideally CaMnBe2Si5O13(OH)2·2H2O, as confirmed by the crystal structure determination. D(meas.) 2.64(1) g/cm3, D(calc.) 2.657 g/cm3. The mineral is orthorhombic, P21ab, with a = 8.729(5), b = 31.326(11), c = 4.903(2)Å, Z = 4. The most intense lines in the powder pattern (Gandolfi camera) are (as d, Å, I/I0, hkl) 15.7(100)(020), 4.15(30)(041), 3.93(30)(080), 3.82(30)(240), 3.28(75)(201), 2.903(100)(251), 1.944 (30)(3.12.0). The genesis is related to late fluid activity within the pegmatite.

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