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A New Mineralspeciesfromthe Tip Top Pegmatite, Custer County, South Dakota, and Its Relationship to Robertsite'

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A New Mineralspeciesfromthe Tip Top Pegmatite, Custer County, South Dakota, and Its Relationship to Robertsite' Canadian Mineralogist Vol. 27, pp. 451-455 (1989) PARAROBERTSITE, Ca2Mn~+(P04h02.3H20, A NEW MINERALSPECIESFROMTHE TIP TOP PEGMATITE, CUSTER COUNTY, SOUTH DAKOTA, AND ITS RELATIONSHIP TO ROBERTSITE' ANDREW C. ROBERTS Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario KIA OE8 B. DARKO STURMAN Departmentof Mineralogy,Royal OntarioMuseum, Toronto, OntarioM5S 2C6 PETE J. DUNN Departmentof MineralSciences,SmithsonianInstitution,Washington,D.C. 20560,U.S.A. WILLARD L. ROBERTS* South DakotaSchoolof Minesand Technology,Rapid City, South Dakota57701,U.S.A. ABSTRACT SOMMAIRE Pararobertsite is rare, occurring as thin red transparent La pararobertsite, espece rare, se presente sous forme single plates, or clusters of plates, on whitlockite at the Tip de plaquettes isolees minces et rouge transparent, ou en Top pegmatite, Custer County, South Dakota. Associated agregats de telles plaquettes, situees sur la whitlockite dans minerals are carbonate-apatite and quartz. Individual crys- la pegmatite de Tip Top, comte de Custer, Dakota du Sud. tals are tabular on {100}, up to 0.2 mm long and are less Lui sont associes apatite carbonatee et quartz. Les plaquet- than 0.02 mm in thickness. The streak is brownish red; tes, tabulaires sur {IOO}, atteignent une longueur de 0.2 luster vitreous; brittle; cleavage on {I oo} perfect; indiscer- mm et moins de 0.02 mm en epaisseur. La rayure est rouge nible fluorescence under long- or short-wave ultraviolet brunatre, et l' eclat, vitreux; cassante; clivage {100} par- light; hardness could not be determined, but the mineral fait; fluorescence non-discernable en lumiere ultra-violette apparently is very soft; D (meas.) 3.22 (4), D (calc.) 3.22 (onde courte ou longue); la durete, quoique indeterminee, (for the empirical formula) or 3.21 g/cm3 (for the ideal- semble tres faible; Dmes 3.22(4), Deale 3.22 (formule empi- ized formula). Pararobertsite is biaxial negative, ex1.79(1), rique) ou 3.21 (formule ideale). La pararobertsite est biaxe (3 1.81(1), 1.83(1), 2 Vx (meas.) 84 (2)°, 2 Vx (calc.) 85°, 1.83(1), 2Vx (mesure) 'Y negative, ex 1.79(1), (3 1.81(1), 'Y Y = band Z /\ c = 4 (2)°in the acute {3angle.The 84(2)°, 2Vx (calcule) 85°, Y = b, Z /\ c = 4 (2)° dans pleochroism is weak; Z = Y reddish brown, X yellowish l'angle aigu {3.Le pleochroi"sme est faible, Z = Ybrun rou- brown. Absorption: Z = Y > X. Microprobe analyses give geatre, X brun jaunatre. Absorption: Z = Y > X. Les Fe203 0.1, Mn203 38.0, CaO 18.3, P20S 35.2, H20 [8.4], analyses it la microsonde electronique ont donne Fe203 sum 100.0 wt.%, which yield the empirical formula 0.1, Mn203 38.0, CaO 18.3, P20S 35.2, H20 [8.4], total Ca8..0s(Mnft87F~.t3h;!1.90(PO.v12.240N4.11.5H20 on the 100.00/0 en poids, ce qui mene it la formule basIs of 68 or, Ideally, Ca2Mn3 (P04h02.3 H20, Ca8.os(Mnft87F~.t3b1.90(P04)12.2407.54.11.5 H20 sur un ° = with Z = 4. The symmetry is monoclinic, space-group base de 68 atomes d'oxygene ou, idealement, P2I/c, with a 8.825(3), q, 13.258(4), c 11.087(3) A, (3 Ca2Mn~+ (P04)302.3H20, avec Z = 4. La symetrie est 101.19(4)°, V 1273 (1) A3, a:b:c 0.6656:1:0.8363. A monoclinique, groupoe spatial PZl/c, a 8.8~5(3), b pronounced A-centered subcell has b' = Ih b. The unit-cell 13.258(4), c 11.087(3) A, (3101.19(4)0, V 1273(1) A3, a:b:c data are given with the unconventional cell setting a < c in 0.6656:1:0.8363. Une sous-maille centree sur A evidente order to facilitate a crystallographic and chemical compar- possede b' = V2b. Les parametres reticulaires sont expri- ison between it and robertsite. The strongest nine repec- mes en termes d'axes non-conventionnels, a <c, afin de tions of the X-ray powder-diffraction pattern [d in A (1) faciliter la comparaison cristallographique et chimique avec (hkl)] are: 8.69(100)(100), 5.66(60)(021), 5.44(50)(002), la robertsite. Les neuf raies les plus intenses du cliche de 3. 179(50),i023), 2.884(60)(300), 2.834(50)(123,042), poudre [d en A(l)(hkl)] sont: 8.69 (100)(100), 5.66(60)(021), 2.772(70)(142),2.611(60)(142),2.163(60)(400). The mineral 5.44(50)(002),3.179(50)(023),2.884(60)(300),2.834(50)(123, is named for its close relationship to robertsite. 042),2.772(70)(142), 2.611 (60)(142), 2.163 (60)(400). Le choix du nom souligne Ie lien etroit qui existe avec la Keywords: new mineral species, pararobertsite, calcium robertsite. manganese phosphate oxide hydrate, Tip Top pegma- (Traduit par la Redaction) tite, Custer County, South Dakota, X-ray data, chem- ical composition, robertsite. Mots-cMs: nouvelle espece minerale, pararobertsite, phos- phate et oxyde de calcium et de manganese hydrate, peg- IGeologicai Survey of Canada Contribution Number matite de Tip Top, comte de Custer, Dakota du Sud, 33488. donnees de diffraction X, composition chimique, "'Deceased March, 1987. robertsite. 451 452 THE CANADIAN MINERALOGIST closely related to robertsite, yet to be different, and they were set aside for several years in the hope of finding additional material. None has been found, despite an assiduous search, and we therefore present the description of the mineral here. The mineral has been named pararobertsite for its close chemical and crystallographic relationship to robertsite. The mineral and name have been approved by the Com- mission on New Minerals and Mineral Names, IMA. Holotype specimens are deposited in the Smithso- nian Institution, Washington, D.C. (NMNH #149606) and in the Royal Ontario Museum (M43771). OCCURRENCE Pararobertsite occurs in secondary seams at the Tip Top pegmatite, near Custer, Custer County, South Dakota, as single platy crystals 0.02 mm thick and up to 0.2 mm long and in clusters (Figs. 1,2). These crystals occur on whitlockite that encrusts FIG. 1. Scanning electron photomicrograph of a group of several generations of carbonate-apatite, which itself pararobertsite crystals. illustrating the typical habit. encrusts quartz. The exact position within the peg- matite is unknown. The mineral is rare at this, the type and only known locality. PHYSICAL AND OPTICAL PROPERTIES Pararobertsite is found as red vitreous transpar- ent plates, up to 0.2 mm in longest dimension, that do not exceed 0.02 mm in thickness. The crystals are tabular on {tOO}. Other forms evident in Figure 1 are too small to measure on an optical goniometer. The mineral possesses a brownish red streak and a perfect cleavage on {100}. The fluorescence is indiscernible under both long- and short-wave ultraviolet light. The crystals are brittle; the hard- ness could not be determined owing to the small size of the individual crystals and to the lack of suffi- cient material, but apparently is very soft. The crys- tals scratched rather easily when touched with a nee- dle. The measured specific gravity, determined using heavy liquid techniques, is 3.22 (4); the calculated density for.the empirical formula is 3.22 g/cm3 and, for the idealized formula, 3.21 g/cm3. Optical properties were determined with a spin- FIG. 2. Scanning electron photomicrograph of the same dle stage and white light on crystals previously group of pararobertsite crystals shown in Fig. 1. rotated studied and oriented by X-ray precession single- by 90°. crystal methods. White light was used for measure- ment in order to enhance the Becke line (which for INTRODUCTION this red-colored mineral is virtually invisible in the reddish, high-index-of-refraction oils). In addition, As part of a continuing investigation of rare the mineral reacts with immersion oils of index minerals found at the Tip Top pegmatite in South greater than 1.80; the Becke line is visible for only Dakota, one of the authors, the late Willard L. a short duration after the crystal is immersed in the Roberts, for whom robertsite was named (Moore & oil. Pararobertsite is biaxial negative, with ex1.79(1), !to 1974), noticed the occurrence of tiny red crys- (3 1.81(1), 'Y1.83(1), 2 Vx (meas.) 84(2)°,2 Vx (calc.) tals. A preliminary investigation showed them to be 85°, Y = band Z 1\ c = 4(2)° in the acute (3angle. PARAROBERTSITE, TIP TOP PEGMATITE 453 In immersion oils the mineral appears dark brown, TAmE 1. X-RAY PG-IDER-DIFF'RACTION DATA FOR PARAroBER'ISITE, and only thin plates are transparent. Pleochroism is CXMP1\RED wrrn DATA EOR OOBER!'SITE P ARAROBERTS[TE ROBERTSITE weak, Z = Y reddish brown, X yellowish brown. (This Study) (Moore &:Ito 1974) The absorption is Z = Y > X. Dispersion of the lest. dAmeas. dkalc. hkl lest. d!\meas. hId optic axis angle could not be observed because the 100 8.69 8.66 100 100 8.63 200 individual grains are too small to provide a 60 5.66 5.66 021 50 5.61 031 50 5.44 5.44 002 conoscopic figure. 3 4.97 4.97 121 10 4.53 4.53 121 20 4.33 4.33 200 20 4.32 500 5 4.11 4.11 210 10 4.14 331 CHEMICAL COMPOSITION 20 3.76 3.76 202 3 3.62 3.62 221 20 3.48 40 3.315 3.315 040 20 3.27 m,B3 Owing to the paucity of material, pararobertsite 50 3.179 3.181 023 40 3.17 160 10 3.095 3.095 140 was chemically analyzed by means of an electron 5 3.053 3.051 113 10 3.03 260 3.024 212 3 3.013 microprobe, utilizing an operating voltage of 15 kV 3.010 213 10 2.937 233 60 2.884 2.886 300 30 2.876 600 and a sample current of 0.025 pA, measured on 2.834 123 50 2.834 brass.
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