Canadian Mineralogist Vol. 19, pp. 393-396 (1981) COMANCHEITE,A NEW MEBGURYOXVCHLORIDE - BROMIDE FROMTERLINGUA, TEXAS A.C. ROBERTS eNn H.G. ANSELL Geologicalsurvey of Canada,60l Booth street, ottawa, ontario KlA 088 P.J. DUNN Depdrtmentol Mineral Sciences,Sntithsonian Instittttion, Washington, D.C. 20560, US'A' ABSTRACT en lumibre ultraviolette.Extinction paralldle.allon- gementpositif. Les indicesde r6fraction se situent '1..79. Comancheite is a new mercurv oxychloride- entre 1.78 et Densit6 mesur6e7.7G). cal- bromide mineral from the Mariposa mine. Terlingua cul6e 8.0. A la microsonde6lectronique' on trouve district, Texas. Associated minerals are calcite, la formule Hg,r(Clr.o,Brs.on)t".nrOo.n".d'oi la for- goethite, hematite and quartz. Comancheite occurs mule id6alisdeHgrs(Cl,Br)nOo. La comanch€iteest as anhedral crystalline massesand as stellate groups orthorhombique.groupe spatialPnnm ou Pnn2. a of acicilar crystals, elongate parallel to c. a\reraging 18.4t(l), b 21.64(l),c 6.677(21A', z = 4' Les 80 um lons and 3 to 4 um wide. Masses are red seot raies les plus intensesdu clich6 de poudre (d with an orange-yellow streak and have a resinous (A), / sur 6chellede l0) sont: 5.68(7).5.42(6), lustre; crystals are orange-red to vellow. vitreous 2.878(8). 2.71I(il, 2.669(10). 2.4s7(5\ et and nanslucent to transparent. Comancheite is 1.415(5). brittle with fair cleavase parallel to {001} and (Traduit Par la R6daction) not {ll0}, has a Mohs hardness of 2 and does de lieht. Opticallv. coman- Mots-clds: comanch6ite,oxychlorure-bromure fluoiesce in ultraviolet mine Mari- exhibits parallel extinction and is length- mercure. nouvelle esdecemin6rale. cheite Texas. fast with all indices of refraction between 1.78 and posa, disffict de Terlingua, 1.79. The measured densitv is 7.7(4): calculated density is 8.0 Mg m-3. Electron-microprobe anal- lNTRODUcrIoN ysis yielded the chemical formula Hg,".no(Clo.ot Brs.so)>a.orOg.or,calculated on Hg = 13. The ideal For nearly a century the Terlingua district of formula is Hgrr(Cl,Br)Oe. Comancheite is ortho- Texas has been worked for its mercury de- rhombic, spac€ group Pnnm or Pnn2, a 18.41(1)' posits; since the discovery of terlinguaite in b 21.64(l\. c 6.677(2) A. a;ndZ = 4. The strons- 1980), it has becomemineralogi- powder pattern tAll {Sttarpu est seven reflections of the X-ray cally well known for its secondary mercury I on a ten-point scale) are: 5.68(7), 5.42(6\' kl L, The Terlingua atea is the type locality 2.7rt(s'), 2.669(10), 2.457(s\ and minerals. 2.878(8), eglestonite (Moses 1.415(5). for terlinguaite, montroydite, 1903), kleinite (Hillebrand & Schaller l9O9), Keywords: comancheite, mercury oxvchloride- mosesite (Canfield et al. l97O), pinchite (Stur- bromide. new mineral, Mariposa mine. Terlingua man & Mandarino 1974) and gianellaite(Tunell '1977). district. Texas. et al. Other mercury minerals found here are cinnabar, calomel and mercury (Yates SoMrtrlrnn & Thompson1959). (ACR) was en- nouvelle espice minErale de la ln 1977 one of the authors La comanch6ite, of mine Mariposa, district de Terlingua (Texas). est un gaged in the routine X-ray identificati-on oxychlorure-bromure de mercure. En association iom" m"t"rrty minerals in the National Mineral avec calcite, goethite, h6matite et quartz. elle se Collection housed at the Geological Survey of pr6sente en amas x6nomorphes et en groupes ra- Canada. One specimenlabeled montroydite from di6s de cristaux aciculaires allong6s selon c, de Terlingua, Texas (purchased in 1964 from dimensions 80 x 3-4 pm. I,es 6chantillons massifs Ward'i Science Establishment, Inc., Rochester, rEsineux: sont rouges, i rayure iaune orangg et 6clat New York) has some reddish massesand mi- du rouge-orange au iaune. ils les cristaux Dassent croscopic crystals that yield an X-ray-diffrac- vitreux et sont translucides d transpa- ont l'6clat powder pattern matching no pattern listed rents. La comanch6ite est fraeile. avec clivage assez tion for inorganic compounds' net suivant {001} et {110}; elle possddeune du- in the JCPDS file ret6 Mohs de 2 et ne montre aucune fluorescence Electron-microprobe analysis and single-crystal 393 394 THE CANADIAN MINERALOGIST X-ray study show that the mineral is a new brown calcite. Minor amounts of goethite, species. This mineral was later found in a hematite and quartz are associated with the 19arly.identical specimen,also labeled rnontroy- comancheite.Comancheite should be considered dite, in the David Wilson mineral collection a rare speciesat the Mariposa mine; to date only now at the Smithsonian Institution. The localitv two specimens are known, and together these given for the second specimen is the Mariposa contain less than one gram of the mineral. How- mine, Terlingua, Texas. No montroydite was ever, close examination of other supposedmon- found on either specimen. troydites from the Terlingua district may lead We take pleasure in naming the new mineral to the discovery of more comancheite. comancheite (KO-MAN,-CHE-ITE) in honor of the first miners in the Terlingua district, the PHysrcer, eNp pnopsntrss Comanche Indians, who used cinnabar for war_ Oprtcal paint. The mineral and mineral name have been Comancheite occurs as anhedral red crvstal- approved by the Commission on New Minerals line nrassesthat have a re$inous lustre and as and Mineral Names, I.M.A. Type specimens are translucent to transparent crystals that are or- preserved in the National Minerai Collection. ange-red to yellow with a vitreous lustre and an Geological Survey of Canada, Ottawa (GSC orange-yellow str€ak. Crystals are acicular, 14608) and the SmithsonianInstitution collec- elongate parallel to c, and approximately 80 tions, Washingron (NMNH 150760). pm in average length with a width of 3 to 4 pm (Fig. l). No indexablecrystal facescould Occunnrrucr be distinguished. Comancheite is brittle and has fair cleavageparallel to {001} and {110}. It has Comancheite is a secondary mineral found at a Mohs hardnessof 2 and does not fluorescein the Mariposa mine, Terlingua district, Brewster ultraviolet light. Comancheite is unaffected by County, Texas, U.S.A. It occurs either as tiny cold concentrated HCl, HrSOn or HNOg. but crystalline masses or, more rarely, as stellate slowly turns a dull orange-brownin 40% crystal groups perched KOH on colorless skeletal cal- solution. The measureddensity, determined with cite, on a matrix yellow- of colorless to banded a Berman balance on 7.5 mg of hand-picked sample,is 7.7(4) Mg m-'. Optically, comancheite exhibits parallel ex- tinction and is length-fast. All indices of re- fraction lie between 1.78 and 1.79. These are significantly lower than the range of indices of refraction predicted from the Gladstone-Dale relationship( approximately2.1 -2.3) ; the reason for the discrepancy is unknown. The strong absorption and small size of the crystals made further optical observationsimpossible. CHsvrcnr CoMposltloN A sample of comancheite in polished thin section was chemically analyzed at the Smith- sonian Instittrtion with an ARL-SEMe electron microprobe utilizing an operating voltage of 15 kV and a beam current of 0.15 p,A. Stand- ards used for analysiswere montroydite (Hg), scapolite (Cl) and synrheticKBr (Br). Oxygen was calculated assuming divalent Hg. A wave- Iength-dispersionmicroprobe scan did not reveal other elementswith atomic number greater than 9. Comancheite was found to be homogeneous over 5 pm sample spots; the reported analytical results (Table 1) were obtained using a 2O pm beam spot in order to minimize evaporation of Frc.,1. Scanning-eleotron photomicrograph of aci- Cl and Br. Analytical data are accurate to 5/o cular comancheite crystals. of the amount pressnt. COMANCHEITE, A MERCURY OXYCHLORIDE-BROMIDE 39s b 21.64(l). TABLEI..CHEMICAL COMPOSITION OF COMANCHEITE ing was possible,gave a 18.41(1), = 2660.1A' and a:b;c : 'ldea l ,k.enQ) A, v l,lt,% 100% atomsln unlt fomula 0.851:1:0.309. Zero' and upper-level single- cell, Hg=I3.00 Cl:Br=9r7 crystal films (both precessionand Weissenberg) Hgo 89,7 *.21 0.4072 Hs l3.00 Hso 88.3 were used to facilitate indexing of the powder cl 5.1 5.01 0.1413 C1 4.51I e 61 c] 5.0 - Br 8.9 8.75 0.1095 Br 3.50'-'-' Br 8.8 pattern. With Z 4, the calculated density, To-tJ 1IT.97 based on the analytical formula (Table 1), is 0.1230 0 9.07 less o=(clrBr) 2.0 1.97 8.0 Mg m-3, in reasonable agreement with the total 101.7 100.00 measured result. The available crystals of comancheite are too X-Rev DIFFRAcTIoN small and diffract too weakly for any meaning- ful crystal-structure determination, especially Of the numerous comancheite crystals ex- considering the large size of the unit cell and amined, only one needle-shapedcrystal about the numbir of atoms contained therein (J'T' 0.1 mm long proved satisfactory for detailed Szymar(ski,Pers. comm. 1980). single-crystal prec€ssion- and Weissenberg- one camera examination. X-ray exposures of ACKNOWLEDGEMENTS week or more were necessaryto determine unit- cell parameters and systematic extinctions. Co- The authors thank G.Y. Chao for the optical mancheite is orthorhombic, with space grouP determinations, D.A. Walker for the scanning- extinction conditions as follows: (l) hkh no electron photomicrograph and J.T. Szymariski conditions, (2) Ok k + I = 2n, (3) hol: h * for helpful comments and for checking the I = 2n and (4) hko; no conditions.These are possibilily of determining the crystal structure' consistent with either space group Pnnm (58) Th" uuthot. are also grateful to Peter Bayliss or Pnn2 (34).