New Mexico Mineral Symposium — Abstracts

A sulfate mineral of composition intermediate between pickeringite [MgAl₂(SO₄)₄.22H₂0] and apjohnite [MnAl₂(SO₄)₄.22H₂O] occurs as an efflorescence in mines at Cunningham Hill in the Ortiz Mountains. It was found at the old "Dolores Tunnel" adit and in an exploratory incline sunk about 1976 at the present site of the Ortiz open-pit gold nine of Gold Fields Mining Corp. The mine is in a breccia, thought to represent a volcanic vent, of Tertiary latite that intruded the Mesaverde Formation (Upper Cretaceous). Gold, scheelite (some as pale-yellow bipyramidal crystals as large as 0.5 cm), pyrite, hematite, and magnetite are disseminated in the breccia. The pickeringite has formed where ground water seeps into the mine tunnels along fracture zones.

Pickeringite (Mg), apjohnite (Mn), and halotrichite (Fe) are isostructural and have nearly identical lattice dimensions and powder diffraction patterns. Divalent cation compositions of the mineral from the Dolores Tunnel and the Ortiz Mine are, respectively,about Mg_.51Mn_.44(Fe,Co,Ca)_.05 and Mg_.48Mn_.45(Zn, Co, Cu, Fe, Ca)_.07. These manganoan pickeringites form pink to white crusts of hairlike crystal fibers about 1-2 mm long by 0.01 mm thick and solid masses of parallel fibers as much as 1 cm long. The strongest X-ray diffraction maxima are at d = 4.81 Å (85), and 4.31 Å (55); unit-cell parameters are a = 6.19 Å, b = 24.35 Å, c = 21.26 Å, and B - 100.3°.

This occurrence of manganese-rich sulfates precipitating from waters acidified by the oxidation of pyrite appears to be unusual, but such deposits may be more common than is generally recognized. Similar material has been found at surface outcrops of sulfide veins near Jackass Creek, Lemhi Co., Idaho.