Sieleckiite is a new copper aluminium phosphate discovered at the Mt Oxide Copper Mine, 150 km north of Mt Isa, Queensland, Australia. It occurs with variscite, turquoise, libethenite and minor pseudomalachite in a fracture in a boulder of quartzite and shale. Sieleckiite forms deep sky blue to royal blue spheres up to 0.5 mm in diameter, made up of fibrous radiating crystals between 20 and 100 µm long and 1 to 2 µm wide. The mineral has a very pale blue streak and a pearly lustre on uneven fracture surfaces. Hardness is about 3 and the measured density is 3.02 g cm⁻³. The average of nine electron microprobe analyses gave CuO 32.39, Al₂O₃ 26.57, P₂O₅ 19.42%. Separate analyses gave H₂O 18.1, CO₂ 1.6% (carbonate impurity). These data gave an empirical formula of Cu_3.1Al_4.0(PO4)_2.1(OH)₁₂.1.7H₂O, calculated on the basis of 22 oxygen atoms. The simplified formula is Cu₃Al_4.0(PO₄)_2.1(OH)₁₂.2H₂O. The strongest lines in the X-ray powder diffraction pattern are {d(I)(hkl)}; 9.12(50)(100), 5.06(100)(101), 3.852(100)(111), 3.276(30)(2¯20), 2.827(50)(1¯02,102), 2.460(50)(3¯21). These data were indexed on a triclinic cell with a 9.41(8), b 7.56(5), c 5.95(6) Å, α 90.25(12)° β 91.27(12)° γ 104.02(7)° and a volume of 410.8(5) ų. For Z = 1, the calculated density is 2.94 g cm⁻³. Optical properties could not be determined in full; the refractive indices are between 1.63 and 1.66, pleochroism is very weak from colourless to very pale blue.

The mineral is named for the discoverer, Robert Sielecki (1958- ). Type specimens are preserved at the Museum of Victoria and the South Australian Museum. Sieleckiite was approved by the IMA Commission on New Minerals and Mineral Names prior to publication.