Phosphovanadylite, whose simplified formula is (Ba,Ca,K,Na), [(V,Al) 4 P 2 (O,OH) 16 ] . 12H 2 O, is a new vanadium phosphate zeolite mineral found in the Phosphoria Formation at Monsanto's Enoch Valley Mine, Soda Springs, Idaho. Its formula in more detail is (Ba (sub 0.38) Ca (sub 0.20) K (sub 0.06) Na (sub 0.02) ) (sub Sigma 0.66) [P 2 (V (sub 3.44) A (sub 10.46) ) (sub Sigma 3.90) O (sub 10.34) (OH) (sub 5.66) ] . 12H 2 O). The drusy mineral occurs as pale greenish-blue euhedral cubes (20-50) mu m edge) coating phosphatic, organic-rich mudstone. The chemical composition determined by electron microprobe is (in weight percent) V-28.02, P-9.91, A1-1.97, Ca-1.31, Ba-8.28, Cd-0.09, Zn-0.34, Na-0.15, K-0.73, O-46.57, and F-0.03. The index of refraction is n D = 1.566 (4) and specific gravity is 2.16 (3). The X-ray powder pattern shows strong reflections at 3.16 Aa (422), 2.58 (600), 2.44 (620), and 7.73 (200), which are indexed on the basis of a cubic body-centered unit cell with a = 15.470 (4) Aa. From the single-crystal structure analysis, its space group was determined to be I43m, Z = 6, and its structure consists of V 4 O 16 octahedral clusters linked to each other by P atoms to form a cubic lattice, creating cavities 7.0 and 5.5 Aa in diameter where mainly H 2 O resides. Final residual indexes are R = 0.066, R w = 0.061, goodness-of-fit = 0.75, and 93 observations and 24 parameters.