Description and crystal structure of turtmannite, a new mineral with a 68 angstrom period related to mcgovernite

Jacobsite-rich Fe-Mn ores of probable Dogger age fill paleokarst pockets in the Triassic marbles of the Barrhorn Unit under Pipjigletscher in the Turt manntal, Valais, Switzerland. These ores and embedding rocks underwent Tert iary metamorphism under upper greenschist facies conditions. Some of these jacobsite ores contain minor amounts of a yellow micaceous mineral, which a ppears, to be a new Mn-Mg silicate-vanadate-arsenate that was named "turtma nnite" with respect to the type locality. Turtmannite flakes up to 200 mum in length occur parallel to the main schistosity, or fill thin discordant v einlets. Turtmannite is rhombohedral R (3) over barc, with a(H) = 8.259(2) and c(H) = 204.3(3) Angstrom in the hexagonal setting. The corresponding pr imitive rhombohedral cell has a(R) = 68.31 and alpha (R) = 6.92 degrees. HR TEM images indicate that turtmannite is perfectly ordered along c. The structure of turtmannite has been solved to a final R1 of 12.4% on a Si emens Smart CCD diffractometer with MoK alpha X-radiation, and a detector t o sample distance extended to 12 cm. The structure consists of 84 oxygen la yers stacked along c, with twelve close-packed layers followed by two non-c lose-packed layers. This sequence is repeated six times. The structure cont ains eight symmetrically distinct cation layers. Three different occupation al variants have been recognized leading to the following hypothetical end- member formulae and approximate abundances: I (Mn1.5Mg3[VI])-Mn-[IV]-Mg-[IV](Mn,Mg)(21)[(V,As)O-4](3)[SiO4](3)O-5(OH)(2 0), 50% II Mn-[IV](1.5)[VI](Mn,Mg)(21)[(V,As)O-4](3)[SiO4](3)[AsO3](OH)(21) 33% III Mn-[IV](1.5)[VI](Mn,Mg)(21)[(V,As)O-4](3)[SiO4](2)[SiO3OH](OH)(25) 16% The simplified chemical formula for turtmannite can be written as: (Mn,Mg)(22.5)Mg3-3x[(V,As)O-4](3)[SiO4](3)[AsO3](x)O5-5x(OH)(20+x). The unit cell of turtmannite is similar to those of mcgovernite, a Mn-Mg-Zn arsenate from Sterling Hill, New Jersey, and an unnamed "mcgovernite-like" Mn-Mg arsenate from the Kombat Mine, Namibia. The crystal structure of tur tmannite is close to the model predicted for mcgovernite by Moore and Araki (1978).