Description: The new mineral whelanite, Cu 2 Ca 6 [Si 6 O 17 (OH)](CO 3 )(OH) 3 (H 2 O) 2 , was approved by the Commission on New Minerals and Mineral Names (IMA) in 1977, but until now a description has not been published. The mineral is orthorhombic with space group Pn 2 n and cell parameters a = 5.6551(4), b = 3.683(3), c = 27.1372(7) Å, V = 565.3(5) Å 3 , and Z = 1. The mineral occurs with thaumasite, stringhamite, and kinoite in a copper-rich, diopside–garnet–magnetite skarn at the Bawana mine, Beaver County, Utah, and has also been confirmed to occur at other localities. At the Bawana mine, it is found as irregular clusters and radial aggregates of platy to lath-like crystals up to 1 mm in length, flattened on {001} and elongated on [100]. The color and streak are pale blue and the luster is vitreous. The laths are flexible, but not elastic. Cleavage is perfect on {001} and good on {010}, producing splintery fracture. The Mohs’ hardness is about 21/2. The measured density is 2.74(3) g/cm 3 and the calculated density is 2.737 g/cm 3 based upon the empirical formula. The mineral is biaxial (–), α = 1.612(2), β = 1.622(calc), and = 1.626(2), and 2 V meas = 64(1)°. The pleochroism is weak: X = Y (pale blue) 〈 Z (light blue). The optical orientation is X = a , Y = c , Z = b . A combination of electron microprobe analyses and thermogravimetric analyses (for CO 2 and H 2 O) yielded: CaO 34.46, CuO 12.09, FeO 1.52, SiO 2 37.96, CO 2 5.93, H 2 O 8.86, total 100.82 wt%, providing the empirical formula (based on O = 26): Cu 1.41 Fe 0.20 Ca 5.68 Si 5.84 C 1.25 O 26 H 9.09 . Raman spectroscopy shows evidence of (CO 3 ) 2– , (OH) – , and H 2 O. The strongest powder X-ray diffraction lines are [ d ( hkl ) I ]: 6.79(004)52, 3.072(111)43, 3.013(112)100, 2.921(113)39, 2.802(114)45, 2.522(116,205)44, and 1.839(020,1.1.12)37. The crystal structure ( R 1 = 3.89% for 567 F o 〉 4 F ) contains two different types of polyhedral layers parallel to {001}, which alternate along [001] and are linked to one another by sharing corners with wollastonite-like silicate chains running parallel [010]. One polyhedral layer, consisting of edge-sharing CaO 7 polyhedra, is identical to that in the structures of the tobermorites (tobermorite 9Å, tobermorite 11Å, tobermorite 14Å, and clinotobermorite). The other layer is brucite-like, with alternating ribbons of edge-sharing Cu 2+ O 6 and CaO 6 octahedra. Disordered CO 3 and H 2 O groups are also located in the interlayer region. The crystal structure of whelanite exhibits OD character.