Marchettiite, (NH4)C5H3N4O3, a new organic mineral from Mount Cervandone, Devero Valley, Western-Central Alps, Italy

The new mineral marchettiite (IMA2017-066) is the natural equivalent of ammonium hydrogen urate. It has a simple molecular formula C5H7 N5 O3 and can be alternatively written as (NH4 )C 5H3 N4 O3 . Marchettiite was found in a cleft at Mount Cervandone, Devero Valley, Piedmont, Italy, where it occurs as aggregates of opaque pale pink to white, platy prismatic crystals. This mineral has a white streak, dull and opaque lustre, it is not fluorescent and has a hardness of 2–2.5 (Mohs’ scale). The tenacity is brittle and crystals have a good cleavage parallel to {001}. The calculated density is 1.69 g/cm 3 . Marchettiite is biaxial (–) with 2V of 47.24°; the optical properties of marchettiite were determined by periodic-DFT methods providing the following values: α = 1.372, β = 1.681 and γ = 1.768. No twinning was observed. Electron microprobe analyses gave the following chemical formula: C 4.99 H6.97 N4.91 O3.00 . Although the small crystal size did not allow refinement of structural data by single-crystal diffraction, we were able to refine the structure by powder micro X-ray diffrac- tion. Marchettiite has space group P 1 and the following unit-cell parameters: a = 3.6533(2) Å, b = 10.2046(7) Å, c = 10.5837(7) Å, α = 113.809(5)°, β = 91.313(8)°, γ = 92.44(1)° and V = 360.312 Å3 . The strongest lines in the powder diffraction pattern [d in Å (I )(hkl)] are: 9.784(50)(001); 8.663(80)(01 1); 5.659(100)(011); 3.443(100)(10 1); 3.241(70)(003) and 3.158(100)(1 1 1). Marchettiite is named after Gianfranco Marchetti, the mineral collector who found this mineral.