American Mineralogist, Volume 86, pages 1494–1505, 2001 0003-004X/01/0009–1494$05.00 1494 * Present Address: South Australian Museum and Adelaide University, North Terrace, 5000 Adelaide, Australia. E-mail: Brugger.Joel@saugov.sa.gov.au Description and crystal structure of turtmannite, a new mineral with a 68 Å period related to mcgovernite JOËL BRUGGER, 1, * THOMAS ARMBRUSTER, 2 NICOLAS MEISSER, 3 CLIVIA HEJNY, 2 AND BERNARD GROBETY 4 1 VIEPS, Department of Earth Sciences, P.O. Box 28E, Monash University, VIC-3800, Australia 2 Laboratorium für Chemische und Mineralogische Kristallographie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland 3 Musée Géologique Cantonal & Laboratoire des Rayons-X, Institut de Minéralogie, UNIL-BFSH2, CH-1015 Lausanne-Dorigny, Switzerland 4 Institut de Minéralogie et Pétrographie, Université de Fribourg, Pérolles, CH-1700 Fribourg, Switzerland ABSTRACT Jacobsite-rich Fe-Mn ores of probable Dogger age fill paleokarst pockets in the Triassic marbles of the Barrhorn Unit under Pipjigletscher in the Turtmanntal, Valais, Switzerland. These ores and embedding rocks underwent Tertiary metamorphism under upper greenschist facies conditions. Some of these jacobsite ores contain minor amounts of a yellow micaceous mineral, which appears to be a new Mn-Mg silicate-vanadate-arsenate that was named “turtmannite” with respect to the type local- ity. Turtmannite flakes up to 200 μm in length occur parallel to the main schistosity, or fill thin discordant veinlets. Turtmannite is rhombohedral R3 – c, with a H = 8.259(2) and c H = 204.3(3) Å in the hexagonal setting. The corresponding primitive rhombohedral cell has a R = 68.31 and α R = 6.92°. HRTEM 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 Siemens Smart CCD diffractometer with MoKα X-radiation, and a detector to sample distance extended to 12 cm. The structure consists of 84 oxygen layers stacked along c, with twelve close-packed layers followed by two non-close-packed layers. This sequence is repeated six times. The structure contains eight sym- metrically distinct cation layers. Three different occupational variants have been recognized leading to the following hypothetical end-member formulae and approximate abundances: I [IV] Mn 1.5 [IV] Mg 3 [VI] (Mn,Mg) 21 [(V,As)O 4 ] 3 [SiO 4 ] 3 O 5 (OH) 20 , 50% II [IV] Mn 1.5 [VI] (Mn,Mg) 21 [(V,As)O 4 ] 3 [SiO 4 ] 3 [AsO 3 ](OH) 21 33% III [IV] Mn 1.5 [VI] (Mn,Mg) 21 [(V,As)O 4 ] 3 [SiO 4 ] 2 [SiO 3 OH](OH) 25 16% The simplified chemical formula for turtmannite can be written as: (Mn,Mg) 22.5 Mg 3–3x [(V,As)O 4 ] 3 [SiO 4 ] 3 [AsO 3 ] x O 5–5x (OH) 20+x . The unit cell of turtmannite is similar to those of mcgovernite, a Mn-Mg-Zn arsenate from Ster- ling Hill, New Jersey, and an unnamed “mcgovernite-like” Mn-Mg arsenate from the Kombat Mine, Namibia. The crystal structure of turtmannite is close to the model predicted for mcgovernite by Moore and Araki (1978). INTRODUCTION The mineralogical study of manganoan ores filling paleokarst pockets that outcrop under the suspended Pipjigletscher (glacier) in the Triassic marbles of the Barrhorn Unit in the Central Alps, Switzerland, has revealed the pres- ence of several As- and V-rich phases. In this communication, we report the description and crystal structure of one of these minerals, which has been accepted by the International Miner- alogical Association as the new mineral “turtmannite.” The holotype (MGL53593) and cotype (MGL58732) are deposited at the “Musée Cantonal de Géologie,” Lausanne, Switzerland. The mineral is named after the Turtmann Valley that hosts the type locality and also has a rich geological background and mining history. Thrust sheet tectonic was discovered in the Turtmann valley (Gerlach 1869; Argand 1911), and several high altitude 19 th century Ni-Co mines are located in the valley (e.g., Plantorin at 3031 m altitude; Schafer 1994). THE MANGANOAN PALEOKARST LENSES AT PIPJIGLETSCHER The Turtmanntal, a tributary to the Rhone Valley in Canton Wallis, Central Alps, Switzerland, is located in the Middle