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(1Eso). the Barite Rock Is Fine-Grained and Has a Gray Color Owing to Varying Amounts of Interspersedshaly Matter

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(1Eso). the Barite Rock Is Fine-Grained and Has a Gray Color Owing to Varying Amounts of Interspersedshaly Matter THE AMERICAN MINERALOGIST, VOL.47, MAY_JUNE, 1962 BENSTONITE, CazBa6(COr)tr,A NEW MINERAL FROM THE BARITE DEPOSIT IN HOT SPRING COUNTY, ARKANSAS Fnrrrnrcs Lrrruenw,l Mineralogische Anstohen der Uniaersildl, Gdttingen,Germany. Ansrnlcr Benstonite, (Ca,Mg,Mn)r(Ba,Sr)6(CO)s, was found at the barite mine in Hot Spring County, Arkansas, where it occurs in veins within the barite body associated with milky quartz, barite and calcite. It forms white cleavable masses which show cleavage faces up to 1 cm across.H:3-4; G 3.596 (meas.), 3.648 (calc.); good rhombohedral cleavagewith about the same angles as calcite. Uniaxial negative with o:1.690s and e:1.527. Single crystal r-ray studies revealed a rhombohedral lattice with the following characteristics: possible space groups RB or R3, the most probable being R3 because of -the symmetry of the CO3-groupand the available equipoints; hexagonal, o:18.28*0.01 A, c:8.67IO.02; rhombohedral, a,t:10.944, a:113.3o; volume 2509 Aa ftex.;, cell contents:3 [CazBao (COa)ul in the hexagonal unit. The index of the cleavageface is [3142] or [4132]. The hexagonal indices of the strongest r-ray reflections satisfy the relation Trzaftzlhk :13n, and they may be transformed to the indices of calcite reflections. The resulting "calcite-ceil" of benstonite has o!:a/t4i3:5.0? A, C':2c:17.34 A; volume 386.0 A3; calcite: a:4.990 A; c:17.06 A; volume 367.80 A3. Benstonite therefore has a structure which may be described as a superstructure derived from the calcite configuration. Ap- proximate parameters for the cation sites in benstonite are derived from the calcite ar- rangement, and four difierent models are proposed for the arrangement of the cations. The chemical analysis was carried out on selected material from which a small ad- mixture of calcite could not be separated. The formula Cae.lsMgo.zsrMno.ognBarzzsSro.zzz (COr)rs calculated after deduction of 2.23 weight per cent calcite may be generalized to CazBao(COa)raor possibly MgCaeBas(COr)n. Benstonite is named in honor of O. T. Benston. Malvern. Arkansas. who drew attention to the mineral. IurnonucrroN In Hot Spring County, Arkansas, abofi 2l miles ENE of the igneous complex of Magnet Cove, a large barite deposit is mined in an open pit operation.The bedsof the depositlie betweenthe Arkansasnovacu- lite (Devonian-Mississippian) and the Stanley shale (Pennsylvanian) at the eastern end of a syncline that follows the strike of the Ouachita mountains. The deposit was describedby Parks and Branner (1933), and the geology of the region is summarized by Fryklund and Holbrook (1eso). The barite rock is fine-grained and has a gray color owing to varying amounts of interspersedshaly matter. In certain placesit contains irregu- Iar white veins, from an inch to a few inches in thickness, in which the new barium calcium carbonate, benstonite, is found. It is associated with milky qvartz, barite and calcite. 1 Present address: Grabbeplatz 12, Dortmund, Germany. J6J FRIEDRICH LIPPMANN Benstoniteis named in honor of O. J. Benston,born 1901,ore dressing metallurgist, Baroid Division, National Lead Company, Malvern, Arkansas. Mr. Benston had guessedfrom qualitative chemical tests and from the specificgravity of about 3.60 that the mineral might be either alstonite or barytocalcite. During a visit to the mine on New Year's Eve, 1954, he kindly furnished the specimenson which the presentstudy is based. Pnvsrc.q.rPnoppnrrBs Benstonite occursas cleavablemasses with cleavagefaces up to one centimeteracross. Cleavage fragments are more or lessperfect rhombo- hedra of the same shape as calcite and the other rhombohedral car- bonates. The cleavageis not quite as perfect as that of calcite. The cleavagefaces are slightly curved, and, especiallyon the larger pieces, they are composite, with mosaic structure as in some dolomite and siderite.On the reflectinggoniometer extremely diffusesignals were ob- served.Values oI 44.7"and 44.3owere obtained for the polar distances (p angles) of the faces on two different cleavagerhombohedra (averages of a large number of readings).For these measuredp anglesthe calcu- lated cleavageangles (tr angles) are 75.I" and 74.4", respectively.(The corresponding literature values of p and tr for calcite are: 44.6" and 74.9".) The hardnessis between 3 and 4. The specificgravity as determined by the pycnometer method is 3.596. The color is snow white to ivory. When irradiated with long or short wave ultraviolet radiation benstonite fluorescesred, which is almost identical with that of a glowing cigaret. The same fluorescenceoccurs when the mineral is exposedto r-rays. Phosphorescenceis perceptibleat 20 kilovolts. At 50 kilovolts the afterglowlasts a coupleof minutes. Oprrcer PnopBnrros In thin sectionbenstonite shows numerous bubbles of about a micron to a few microns diameter and other more irregularly shapedcavities of similar dimensions.These are responsiblefor the white overall appear- anceof the mineral which otherwisewould be translucent.Extinction of individual grainsis not uniform. Domains varying in sizefrom 0.1mm to over 1mm are displaceda few degrees.In somecases the boundariesare sharp, but usually the domains merge almost imperceptibly with each other. This phenomenon,which occurscommonly in siderite and dolo- mite correiateswith the composite nature of the cleavagefaces. It is similar to the undulatory extinction of deformedquartz grains in meta- morohic rocks. BENSTONITE 587 Benstonite yields a uniaxial negative interference figure. Even in thick sectionsthere is no indication that the mineral is biaxial with a small 2V. This is important for the distinction from alstoniteand barytocalcite,for which axial anglesof 6o and 150,respectively, are reported. The refractive indices were determined for sodium light by the im- mersion method: <o:1.690s*0.0005;e:1.527 +0.001. Cleavagefrac- tures 10-100 microns acrossmay be filled by another carbonate mineral, probably calcitebecause of its lower <,r. X-nev CnysrarrocRAPHY A well terminated cleavagerhombohedron about 0.2 mm in size was mounted parallel to its trigonal axis. After orientation by the oscillation method, a rotation photograph and equi-inclinationWeissenberg photo- graphs ol zero, first, and secondlevels were taken with Ni-filtered Cu-K" radiation. All reflections observed satisfy the condition of rhombohedral centering-h+k+l:3nlor hk.l. The lattice levelsymmetries G of the zero Iayer and Cg of the first and secondlayers yield the diffraction sym- bol 3 R- which includesthe spacegroups R3 and ft3. The lattice con- stants weredetermined to be o: 18.4A, from the zero-layerWeissenberg photograph,and c:8.75 A, from the layer-linespacings on the rotation photograph. In indexing the powder data obtained with NaCl as an internal standardo:18.28*0.01 A and c:8.67 +0.02 A gavethe closestfit of calculatedand measuredd-spacings (Table 1). For thesehexagonal lattice constantsthe axialratio is: a:c:l:0.4743. The calculateddimensions of the rhombohedralcell are: a,n:10.94 A and a:113.3o. The cleavage rhombohedronmust have the index {3 | 4 2l or {4 1 3 2}1. For these indicesthe calculatedp angleoI 44.6"(I : 75.0') is within the rangeof the two measuredones. The indicesof the cleavageface are substantiatedby the fact that the corresponding powder diffraction line, which is the strongestof the whole pattern, is of greatly variable intensity with more or less preferred orientation. The cleavage and the r-ray diffraction properties suggestedthat the structure of benstonitemight be related to calcite.It was found that the strongest reflectionsin the powder pattern have hexagonalindices which satisfythe condition h2lk2lhh:13n. Thesemay be subdividedinto two groupswith 4hlk:13n and 3h-k:13n. If the first group is converted by the transformatior'4/fi t/13.0/-I/tt s/1t.0/0 0.2 the set of indices oI r-ray reflections of calcite is obtained. The same result is arrived at when the indices of the second group undergo the transformation I Right- or left-handed, respectively. TAsr.n 1. X-R,w Powonn Dlta lon CAr.crtn (SweNsoN aNo Fuval, 1953) eNn ror BrxsroNrtn,r CazBao(COa)u,Hrxncoxl.t-, o: 18.2810.01 A, c: 8.67+0.02 A, Pnoeanlr Sp.a.cnGnoup R3 (PnnsrN:r Srunv) Strong reflections are underlined. Relative d.oo,L fr Rel. int.a tlo,o, y ^fu do r. hh I3 calcite intensity2 measured synthetic calcite calculated cell 3 9.21 9.140 11.0 /.oJ 7 605 10 1 z 5.83 5.843 02.1 3 5.28 5.277 03.0 / 4.93 4.925 21.1 J 4.57 4.570 22.O t6 4.19 4.181 or.2 38 3.92 3.917 13.1 0t.2 3.86 I 3.80 3 .802 20.2 9 3.60 3 .601 40.r 3.511 12.2 4 3.45 3 .455 14.0 10 353 3 .350 32.1 95 3 085 3 085 3t.2 10.4 3 035 3.O47 33.0 T 2.98 2.974 05.1 2.923 0+.2 I 2.89 2.8q 00.3 00.6 2.845 2.828 24.r 2.784 232 2.755 2.756 11.3 2.702 51,1 I 2.637 2.639 06.0 2 2.557 2.557 50.2 28 2.536 2 535 25.O 11.0 2.495 2.535 03.3 1 2.495 2.493 43.1 1 2.46 2.462 Aa a 9 2.M5 2.M3 22.3 € N 2 .380 2.378 152 N 2.327 2.326 16.1 1 2.281 2.285 440 I Benstonite with NaCl as internal standard, Norelco X-ray spectrometer, ]o per minute, 1o slit, receiving slit 0.2 mm; CuKa radiation, Ni-filter.
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