Denningite, (Mn, Ca, Zn)Te 2 O 5 , was found near Moctezuma. It occurs in small cleavable masses and occasionally in minute euhedral crystals. The ion color ranges from pale green to colorless; the luster is adamantine; hardness is 4; and specific gravity is 5.05. Denningite is optically uniaxial positive; omega = 1.89 and epsilon = 2.00. The mineral is tetragonal with a space group of P4 2 /nbc; a = 8.82 Aa; c = 13.04 Aa; a:c = 1:1.478. The unit cell contains 8 [(Mn, Ca, Zn)Te 2 O 5 ]. Strongest X-ray powder spacings are: 4.42 Aa (VS), 3.38 Aa (S+), 3.12 Aa (S), 2.62 Aa (S), and 2.03 Aa (S). Denningite is associated with tellurite, paratellurite, native Te, spiroffite, and several other new tellurites. The name is given in honor of Reynolds M. Denning, Professor of Mineralogy at the University of Michigan.