THE AMERICAN MINERALOGIST, VOL.47, MAY_JUNE, 1962

BENSTONITE, CazBa6(COr)tr, A NEW MINERAL FROM THEBARITE 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 SpringCounty, Arkansas, where it occurs in veins within the barite body associated with milkyquartz, barite and calcite. It forms white cleavable masses which show cleavage faces upto 1 cm across. H:3-4; G 3.596 (meas.), 3.648 (calc.); good rhombohedral cleavage withabout the same angles as calcite. Uniaxial negative with o:1.690s and e :1.527. Singlecrystal 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 ofthe CO3-group and 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 cleavage face 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 structurewhich 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 toCazBao(COa)ra or possibly MgCaeBas(COr)n.

Benstonite is named in honor of O. T. Benston. Malvern. Arkansas. who drew attentionto the mineral.

IurnonucrroN

In Hot Spring County, Arkansas, abofi 2l miles ENE of the igneouscomplex of Magnet Cove, a large barite deposit is mined in an openpit operation. The beds of the deposit l ie between the Arkansas novacu-lite (Devonian-Mississippian) and the Stanley shale (Pennsylvanian)at the eastern end of a syncline that follows the strike of the Ouachitamountains. The deposit was described by 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 varyingamounts of interspersed shaly matter. In certain places it contains irregu-Iar white veins, from an inch to a few inches in thickness, in which thenew barium calcium carbonate, benstonite, is found. It is associatedwith milky qvartz, barite and calcite.

1 Present address: Grabbeplatz 12, Dortmund, Germany.

J6J

FRIEDRICH LIPPMANN

Benstonite is named in honor of O. J. Benston, born 1901, ore dressingmetallurgist, Baroid Division, National Lead Company, Malvern,Arkansas. Mr. Benston had guessed from qualitative chemical testsand from the specific gravity of about 3.60 that the mineral might beeither alstonite or barytocalcite. During a visit to the mine on NewYear's Eve, 1954, he kindly furnished the specimens on which thepresent study is based.

Pnvsrc.q.r PnoppnrrBs

Benstonite occurs as cleavable masses with cleavage faces up to onecentimeter across. Cleavage fragments are more or less perfect rhombo-hedra of the same shape as calcite and the other rhombohedral car-bonates. The cleavage is not quite as perfect as that of calcite. Thecleavage faces are slightly curved, and, especially on the larger pieces,they are composite, with mosaic structure as in some dolomite andsiderite. On the reflecting goniometer extremely diffuse signals were ob-served. Values oI 44.7" and 44.3o were obtained for the polar distances(p angles) of the faces on two different cleavage rhombohedra (averagesof a large number of readings). For these measured p angles the calcu-lated cleavage angles (tr angles) are 75.I" and 74.4", respectively. (Thecorresponding literature values of p and tr for calcite are: 44.6" and7 4.9" . )

The hardness is between 3 and 4. The specific gravity as determinedby the pycnometer method is 3.596.

The color is snow white to ivory. When irradiated with long or shortwave ultraviolet radiation benstonite fluoresces red, which is almostidentical with that of a glowing cigaret. The same fluorescence occurswhen the mineral is exposed to r-rays. Phosphorescence is perceptible at20 kilovolts. At 50 kilovolts the afterglow lasts a couple of minutes.

Oprrcer PnopBnrros

In thin section benstonite shows numerous bubbles of about a micronto a few microns diameter and other more irregularly shaped cavities ofsimilar dimensions. These are responsible for the white overall appear-ance of the mineral which otherwise would be translucent. Extinction ofindividual grains is not uniform. Domains varying in size from 0.1mm toover 1mm are displaced a few degrees. In some cases the boundaries aresharp, but usually the domains merge almost imperceptibly with eachother. This phenomenon, which occurs commonly in siderite and dolo-mite correiates with the composite nature of the cleavage faces. It issimilar to the undulatory extinction of deformed quartz grains in meta-morohic rocks.

BENSTONITE

Benstonite yields a uniaxial negative interference figure. Even in thicksections there is no indication that the mineral is biaxial with a small 2V.This is important for the distinction from alstonite and barytocalcite, forwhich axial angles of 6o and 150, respectively, are reported.

The refractive indices were determined for sodium light by the im-mers ion method: <o:1.690s*0.0005; e:1.527 +0.001. Cleavage f rac-tures 10-100 microns across may be filled by another carbonate mineral,probably calcite because of its lower <,r.

X-nev CnysrarrocRAPHY

A well terminated cleavage rhombohedron about 0.2 mm in size wasmounted parallel to its trigonal axis. After orientation by the oscil lationmethod, a rotation photograph and equi-inclination Weissenberg photo-graphs ol zero, first, and second levels were taken with Ni-filtered Cu-K"radiation. All reflections observed satisfy the condition of rhombohedralcenter ing -h+k+l :3nlor hk. l . The lat t ice level symmetr ies G of thezero Iayer and Cg of the first and second layers yield the diffraction sym-bol 3 R- which includes the space groups R3 and ft3. The lattice con-stants were determined to be o: 18.4 A, from the zero-layer Weissenbergphotograph, and c:8.75 A, from the layer-l ine spacings on the rotationphotograph.

In indexing the powder data obtained with NaCl as an internals tanda rd o :18 .28*0 .01 A and c :8 .67 +0 .02 A gave the c loses t f i t o fcalculated and measured d-spacings (Table 1). For these hexagonal latticeconstants the ax ia l rat io is : a :c : l :0 .4743. The calculated d imensions ofthe rhombohedral ce l l are: a,n:10.94 A and a:113.3o. The c leavagerhombohedron must have the index {3 | 4 2 l or {4 1 3 2}1. For theseindices the calculated p angle oI 44.6" (I : 75.0') is within the range of thetwo measured ones. The indices of the cleavage face are substantiated bythe fact that the corresponding powder diffraction line, which is thestrongest of the whole pattern, is of greatly variable intensity with moreor less preferred orientation.

The cleavage and the r-ray diffraction properties suggested that thestructure of benstonite might be related to calcite. It was found that thestrongest reflections in the powder pattern have hexagonal indices whichsatisfy the condition h2lk2lhh:13n. These may be subdivided into twogroups wi th 4hlk :13n and 3h-k:13n. I f the f i rs t group is convertedby the transformatior'4/fi t/13.0/-I/tt s/1t.0/0 0.2 the set of indicesoI r-ray reflections of calcite is obtained. The same result is arrived atwhen the indices of the second group undergo the transformation

I Right- or left-handed, respectively.

587

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: 18.28 10.01 A, c: 8.67 +0.02 A,

Pnoeanlr Sp.a.cn Gnoup R3 (PnnsrN:r Srunv)

Strong reflections are underlined.

Relativeintensity2

d .oo , Lmeasured

do r. frcalculated

hh I3 calcitecell

Rel. int.a

synthetic

tlo,o, y ̂ fucalcite

3

z

3/J

t638

I9

4

1095

T

I

I2

28

119

1

1 1 . 010 102.103 .021.122 .Oor.213 .120.240.r1 2 . 214 .03 2 . 13t.233 .005 . 10+ .200 .32 4 . r2 3 21 1 . 35 1 , 106.050.22 5 . O03 .343.1A a a

2 2 . 31 5 21 6 . 14 4 0

0t .2

1 0 . 4

0 0 . 6

1 1 . 0

9 . 2 1/ . o J

5 .835 . 2 84 . 9 34 . 5 74 . 1 93 . 9 23 . 8 03 . 6 0

3 .453 5 33 085

2 . 9 8

2 . 8 9

2 . 7 5 5

2.6372 . 5 5 72 . 5 3 6

2 . 4 9 52 . 4 62 . M 52 .3802 . 3 2 72.281

9 . 1 4 07 6055.8435 . 2 7 74.9254 . 5 7 04.1813 . 9 1 73 .8023 .6013 . 5 1 13 .4553 .3503 0853.O472 . 9 7 42.9232 . 8 q2.8282 . 7 8 42 . 7 5 62 . 7 0 22.6392 . 5 5 72 5352 . 5 3 52.4932.4622 . M 32 . 3 7 82 . 3 2 62.285

3 . 8 6

3 035

2.845

2.495

€N

N

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.2 Relative intensities are peak heights on spectrometer trace. They are distorted by

preferred orientation.3 Only the indices with positive numbers are listed for every difiraction line in the

hk.l, col:trmr'. The other possible (enantiomorphic) index is given under hi'l only when

necessary for the transformation to the "calcite-cel1" of a right-handed crystal.a The indices, intensities, and d-spacings of calcite reflections hk'|, with I odd are written

vertical. They have no equivalent in benstonite because its c-spacing is about hali that of

calcite.

Tasr,e' 1 (continuerl)

Relativeintensity2

d n n t Ameasured

du," t f\calculated

hk . l 3

.)10

4

?

234A

J

?

10

1 1

2 .2332 2172. t89

2 . r 52. t272. t132 .W7

2.O422 .W6t.9941 .973| 957

1 905

I .8861 .8631 .834

t . 7 4 61 . 7 2 8

1 .6981 . 6 9 2r . 6 7 01.6641.645

2.2312.2r72. 1882 . 1 8 82. t482.128

2.r092 . @ 72.0972.O9r2 .0382.0051.995| . 9 7 41 .95S1.949t . 9 M1.9291 .9061 .9011 .8881 .8611 .836I .8281.8 t21 .8001 . 7 9 21 7891 . 7 7 71 . 7 6 6r . 7 5 91 . 7 5 5r . 7 4 5r . 7 2 7t .724L a A

1 . 6 9 7r . 6 9 41 .675t .6661.6481.6421.6421 .6301 6301 .6131 .550

34.2r+.37 0 . 1J 5 I

1 0 . 46 2 . r6t.21 7 . 0J J . J

02.42 1 . 453.23 6 . 05 4 . 12 6 . 20 6 . 313.40 8 . 12 5 . 3

0+.42 7 . r3 2 . 445.255 .08 1 . 18 0 . 244.30 5 . 446.17 2 . 20 9 . 024.47 3 r28.O5 1 . 40 1 . 5l / . J

20.564.21 2 . 53 7 . 247 .O3 6 . 36 5 . 11 S . 13 1 556.2

,7

26

h i . l

28.2

1 4 4

ht kt . I t4

calcitecell

Rel. Int.asynthetic

02.4

0 1 . 8

1 1 . 6

82q

dv6' v A

calcite

2.09s

1 .927

r.913

1 .875

1.604l . ) d /

1 . 5 2 5

N

590 FRIEDRICH LIPPMANN

Tesla 2. CourenrsoN BarwenN DrunNstotts on ,'Cerrrrn-CEtr,,, oF BntqsroNrrrlNl Srnucrunr Cnr,r, or C,r.r,crrn

"Calcite-cell" of benstonite Structure cell of calcite

hexagona l a ' a - : 5 .O7

A" \/13

C ' : 2 c : 1 7 . 3 4 A

2Vv , : _ : 3 8 6 . 0 A

I J

a ' : C ' : l : 3 , 4 2 0index of cleavage iace: {tOf+}rhombohedral a' ,n:6.47 s A

a ' : 4 6 . l o

a :4 .990 i |

c : r7 .06 L l

v :367.80 A3

a ic :1 i3 .4189

{ 1or4 }a,x :6375 Aa:46 .08o

1 Goldsmith and Graf, 1958.

s/ ts - r / rc 'O/ t / rs +/ rc-0/0 0.2. The inverse matr ices are 3- I .0 / l4 . 0 /00. 1 / z and 4 l . 0 / - | 3 . 0 / 0Q. 1/2, respect ive ly . I f the indices of thex-ray refl.ections of calcite with l: 2n are converted by one of the reversetransformations the indices of the strongest benstonite spacings result.This means that the structure of benstonite is based on a pseudo-cell thehexagonal dimensions a' ar'd c'of which are related to the structure celldimensions of benstonit e, a and c, by at : a/ \/ 13 and c' : c. Two adja-cent pseudo-cells in the c-axis direction have total dimensions similar tothe structure-cell of calcite. The numerical data of the "calcite-cell" ofbenstonite are compared with the structure cell data of calcite in Table 2.The axial ratios are almost identical, but the "calcite-cell" of benstonite islarger, as can be expected from the larger size of the barium ions with re-spect to the calcium ions. From this similarity as well as from the simi-larity of the r-ray powder pattern of calcite and the strongest reflectionsof benstonite it may be concluded that the structure of benstonite is ofthe same general scheme as the structure of calcite with the cationsordered to form a superstructure (and perhaps the CO3-groups rolatedwith respect to the calcite configuration).

The pseudo-celi (and also the "calcite-cell") is rotated by 13.9'versusthe real cell. The sense is counterclockwise, if the indices of benstonitewith 4h*k:13n are transformed, or clockwise, if the indices with 3h-k:13n are involved. In the space group R 3 both orientations are identicalif viewed from opposite directions along the c-axis. In the space groupR 3 the two arrangements are enantiomorphic. Since the cleavage face isthe most prominent morphological feature it wii l customarily be indexed

BENSTONITE

Frc. 1. Calcite-type cation layer (calcite-cells: dashed lines, lattice constant o') withright-handed benstonite-type superstructure cell (full lines, lattice constant o'.t/13)containing 13 cations per unit in the layer. Equipoints are numbered according to thesymmetry of R3 as in Table 3, column 1.

l3l42l in right-handed crystals or | j132} in left-handed ones. Whetherthis assignment is really true could only be decided by a determination ofthe absolute configuration. From the customary indexing and the relationto the calcite structure it follows that the strong reflections are assignedindices with 4h*k:13n in the right-handed enantiomorph and with3h- k :13n in the le f t -handed one.

The "calcite-cell" of benstonite (or two pseudo-cells) wil l certainlycontain 6(RCOa) as in the hexagonal structure cell of calcite. Since thetransformation of the structure cell to the "calcite-cell" is accompaniedby a change in volume by the factor 2f 13, there wil l be 39 carbonateunits, i.e.39(RCOr, in the hexagonal structure-cell of benstonite. In thespace groups R 3 and R 3 this cell content implies for a structure similarto calcite that three cations and three carbonate groups have to belocated at unique threefold positions. In the space group R 3 these havethe point symmetry 3. Since this is incompatible with the shape of thecarbonate group, R3 is ruled out. In the space group R3 the three uniquecat ions and anions are on posi t ions (a) 0,0, z wi th z :0and z- l /2 , re-spectively. The remaining 36 cations and 36 anions must be distributed to4 general ninefold positions each, with z-0 and z-l/2 respectively. Theavailable positions in one cation-layer as well as the pseudo-cell relationare sketched in Fig. 1. For a carbonate with two different cations the fol-lowing f ormulae can be expected: XrYu(COa)rs, XaYro(COs)u, X4Ye(COB)rl

592 FRIEDRICH LIPPMANN

T,tsr,B 3. Possrsrr Penewrnns or Cerror.rs rN a Rrcnr-Haronn Supnnsrnucrunnor BnlrsroNrrn Tvpn

x ' y ' z 'calcite-

cell

0 , 0 , 01 , 0 ,01 , 1 , 02 , o , 01 , 2 , 0

y z rModel 1 Model 2 Model 3 Model 4

Benstonite-ceil

3 Car29Rrr9Rrrr9Rw9 R v

0, 0 ,04 /13 ,1 / r s ,o3/ t3,4/13,O8/13,2/ t3,02/13,7 /13,O

9999

3Ca?9 Carr9 Bar9 Barr9 Carrr

3 Caf9Carr9 Carrr9 Bar9 Barr

3Ca12

9 Carr9 Bar9 Carrr9 Barr

1 All positions have been expanded by the symmetry elements of R3.2 Or Mg for MgCaeBao(COa)ra.

and X?Y6(COa)13. Of these, CaTBao(CO3)s agrees very closely with thechemical analysis (see below).

Four difierent arrangements (models) are possible for one enantio-morphic form. They are listed in Table 3 for a right-handed crystal. Thecarbonate groups wil lbe on analogouspositions with z-l / 2.Nopredictioncan be made regarding their orientations, which must be different fromthose in calcite on account of the difierence in the c-spacings. A detailedstructure analysis wil l have to decide between the possible arrangementsof the cations and determine the configuration of the carbonate groups.

Cururcar- CouposrrroN

Selected cleavage fragments were tested for uniform fluorescence to in-sure the absence of barite and quartz. About 10 grams of powder were ob-tained for analysis by wet chemical methods. When this was examined by*-ray dif iraction an additional l ine at 3.03 A with an intensity l ike themedium-weak lines of benstonite appeared. Since this l ine is absent in theother powder diagrams prepared of benstonite it had to be assigned to thestrongest reflection of calcite. This mineral accompanies benstonite andmay be intimately intergrown as was seen during the optical study. In afloat test with bromoform no weighable amount of calcite could beseparated. Some excess of calcium carbonate is, however, to be expectedin the chemical analysis.

Samples of one half to one gram were used for every determination orseparation. After decomposition with hydrochloric acid, carbon dioxidewas absorbed by soda Iime and weighed. Barium was separated andweighed as barium chromate, precipitated from the acidic, acetatebufiered solution. Calcium and strontium were determined together as

Ca?BarBarrCarrCarrr

BIJNSTONITE

Tesr-E 4. Cnnlnclr, Aralvsrs ol BENSToNTTE Merpnr,ll

Analyst: F. Lippmann

593

BaOSrOCaOMsoMnO

43.054 .O2

19.52r . 6 90 . 35

3 1 . 3 599 .98

wt. 96 104-

2807388

348141949

BaCO:SrCO:CaCOrN[gCO;MnCOa

wt .7o t

55 .40. ) . / . t

34.843 . 5 30 . 5 7

Sum of cations

COzTotal

100.07

7 t23

1 Calculated from cation oxides.

oxalates. In one instance both were precipitated as carbonates. These

were dissolved in nitric acid, and from the dried nitrates calcium nitrate

was extracted with a mixture of ether and alcohol. A value oI 4.021s StO

was obtained in this way. This value agrees satisfactori ly with the 38070

SrO determined by r-ray fluorescence on the whole mineral with bromine(KBr) as an internal standard.

Magnesium and manganese were each determined on separate samples.

In the magnesium determination, manganese was first removed with

ammonia and bromine. Then barium, strontium and part of the calcium

were precipitated as sulfates. The remaining calcium was removed as

oxalate. From the final f i l trate magnesium was precipitated as mag-

nesium ammonium phosphate. Before the manganese could be oxidized to

permanganate by persulfate for the colorimetric determination, barium,

strontium and most of the calcium had to be removed as sulfates from the

nitric acid solution.There was no weighable insoluble residue. No lead and no iron were

found with the usual wet chemical tests.The results of the chemical analysis are given in Table 4. The values

are averages of closely agreeing duplicates, except for SrO. Here the wet

chemical value is tabulated because it was deemed more consistent with

the other wet chemical data. There is only a slight excess of the cations

over carbon dioxide as is evident from the quotients of weight percentages

over molecular weights. Consequently, carbonate weight percentages

calculated from the cation oxides and adjusted to 100/s were used for the

computation of the chemical formula presented in Table 5.

594 FRIEDRICH LIPPMANN

Tanr,r 5. Cer,cur,Arrol,r oF TrrE FoRMUT,A lon BnNstoNtrn

wt.'/"twt. Ta tu

Mol. wt

wt.7o lu

Mol. wt. 532Mol. lo

BaCOr

SrCOs

55 .37

5 . 7 2

MgCOa 3 .53MnCOs 0.57

5 273

o . 7 2 7

6,000

0. 7883947 : 7:564 0.094

3724:7 . 532 6.118

223 7,000

:6 :0 .879 40 .56

: 6 : O . l 2 l 5 . 5 9

:7 :0 . l l3 6 .06: 7 : 0 . 0 1 3 0 . 7 2

: 7 : 0 . 8 7 4 4 7 . O 7

28os ilstoz: o-stz

387 )

combined CaCOs

free

34.81 34.81 3478

7139-233:6919

1 From last column of Tabie 4 adjusted to 100/y2 The figures in the box were entered at a later stage of calculation.Formula:

Cao usMgo zrsMno oslBas:zsSro z:z(CO:)ra

or

(Cao szrMgo u:Mno or)z(Bao srgSro tgr)r(COr)rr

Average molecular weight of RCOr:141.35; 39(RCO3) per hexagonal unit cell

V:2509 A3; G(calc.) :3.648.

38.44 (RCO3) per unit cell (5.91 (RCOr) per "calcite-cell") for experimental density

3 596.

In all attempts to calculate a formula for benstonite from the chemicalanalysis, the large cations Ba and Sr and the smaller ones Ca, Mg, andMn were combined in two groups. The discovery that the analyzed pow-

der contained a few per cent of calcite as a discrete phase had to be takeninto account. No quantitative r-ray determination of the amount of cal-cite was attempted because of the complications to be expected from pre-ferred orientation. Before the relation to the calcite structure was recog-nized, a formula of the type XY(COt, was attempted in several varia-tions. First, 5/6 oI free calcite was subtracted for an approximate ratio

1:1 of the two groups of cations. This procedure, however, either yielded

strongly deviating calculated densities or else improbably high (about

l5/) or negative amounts of free calcite had to be presumed for 36 or for

II)

41950

BENSTONITE

42 carbonate molecules, respectively, per hexagonal unit cell. The cellcontent of 39 derived from the pseudo-cell relation necessitates unequalamounts of small and large cations. Their ratio is 7.42:6, without deduc-tion of free calcite. For the ratio 7:6 demanded by the formula compatiblewith the unit ceII and its symmetry, 2.23 weight/o of CaCO3 has to besubtracted from the analysis. The calculated amount of free calciteagrees with the relative intensity of 3.03 A reflection. The resultingformula for benstonite is given under Table 5. The calculated density of3.648 aggrees with the experimental one of 3.596, if the presence of themicroscopical cavities and the possible contamination with calcite aretaken into account. The sum (Mgf Mn) is high enough to permit con-jecture about the concentration of these cations at the unique threefoldpositions of the unit cell. A structure analysis will reveal whether it isjustified to write the formula

or in detail

MgCaoBao(COa)ra

(Mgo. zeaMno. osrCas n6) Ca6(Bas ersSro.Dr)o(COa)ra

Mrscprr,aNrous OBSERVATToNS

When benstonite is crushed the odor of hydrogen sulfide is perceived.fhis compound must form part of the filling of the microscopic cavities.The presence of hydrogen sulfide in the voids of benstonite means thatthe mineral formed in a reducing environment, which was probablycaused by the overlying bituminous Stanley shale. It is likely that thebarium of the benstonite came from the bedded barite, a small part ofwhich was dissolved in the reducing environment, whereas calcium andmagnesium were introduced by percolating waters.

The red fluorescence of benstonite is probably due to its manganesecontent. This conclusion is suggested by the following experiments:Benstonite is dissolved in hydrochloric acid. If the cations are pre-cipitated from the hot solution as a carbonate by addition of ammoniaand ammonium carbonate, the carbonate obtained when exposed to X-rays fluoresces with the same color as benstonite. The intensity howeveris weaker. If the manganese is first removed by precipitation with am-monia and bromine and filtration, the carbonate precipitated from thefiltrate does not fluoresce. No attempt was made to identify the nature ofthe precipitate. According to the experiments of Terada (1953) a rhombo-hedral mixed crystal of barium calcium carbonate should have formed.Therefore the above described experiments are not a direct proof thatmanganese causes red fluorescence in benstonite. They only show thatmanganese can make complex barium-calcium carbonates fluorescent.

596 FRIEDRICH LIPPMANN

Schulman el al. (1947) t have shown that red fluorescence in calcite re-quires the presence of manganese plus a second activator element such aslead, which might be present in benstonite, as a trace element. Otherwisebarium could be the second activator. (See note, p. 598.)

DrscussroN

Two compounds of the formula BaCa(COa)z are known as minerals:barytocaicite, monoclinic, and alstonite, orthorhombic. The structure ofbarytocalcite was determined by Alm (1958). It is ordered with respect toBa and Ca, these cations occupying discrete positions in the structure.Alstonite is very similar to aragonite and witherite both in morphologyand lattice constants. It is not yet known whether alstonite is orderedwith respect to Ba and Ca, but the weak additional layer l ines observedby Gossner and l{ussgnug (1930) on their rotation photographs may betaken as evidence in favor of a superstructure.

Rhombohedral mixed crystal carbonates of barium and calcium weresynthesized by Terada (1952, 1953) with up to 55 mol 7o BaCOy Theyyielded r-ray powder patterns very much like those of calcite. Reflections(hk.l) with I odd were missing, if there were 20 or more mol/6 BaCOs.This means that the lattice constant in the c-direction is about half thatof calcite, just as in benstonite. Apparently the presence of enough Bachanges the articulation of the CO3-groups in calcite-type carbonates.For the composition of 40 mol 7o BaCOt and 60 mol /6 CaCO3, whichcompares most c losely to benstoni le , a:5.06 A and 2c:17.69 A arecalculated from the rhombohedral data of Terada. The o-value isslightly smaller and the 2c-vaIue is higher than for the "calcite-cell" ofbenstoni te (at :5.o7 L, 2r :1734 h) . The cel l vo lume V:390 A3 isidentical with that of the "calcite-cell" of benstonite V':386 At *ithinthe l imit of error.

If compared with the barium-calcium carbonates of Terada, benstonitehas a superstructure. One cause for the ordering of the cations in benston-ite may be its slow rate of formation. If, however, it turns out that theMg-ions in benstonite occupy the special equipoints 0, 0, 0 of the hexag-onal cell, so that the formula is MgCa6Bau(COr)tr, it is more l ikely thatthe presence of magnesium is responsible for the formation of the super-structure. The intervention of magnesium could be the reason why ben-stonite occurs at all, i f one considers the existence of the other bariumcalcium carbonates which do not seem to contain appreciable amounts ofmagnesium according to the analyses reported in the l iterature.

Whereas dolomite, CaMg(COe)2, and perhaps norseth i te BaMg(COr)z

1 Dr. Michael Fleischer kindlv directed the author's attention to this article.

BENSTONITE 597

(Mrose et al. 196l), are superstructures derived from the calcite con-figuration by the distribution of the different cations into alternating cat-ion layers, the superstructure of benstonite is formed by the ordering ofthe cations within the layers. This is accompanied by a different articula-tion of the CO3-groups as is evident from the c-value which is about halfthat of calcite. Another example of a rhombohedral carbonate structurewith about half the c-value (7 .82 A) of calcite and with ordering of the cat-ions within the cation layers is huntite 1\t[g3Ca(COr)a (Graf and Bradley,re62).

It is interesting to note that complex calcite-type carbonates, such asthe mix-crystals of Terada, norsethite, and benstonite, exist with a cat-ion as large as barium. This does not agree with the current explanationthat the crystall ization of CaCOs in the form of aragonite is favored oreven stabil ized by the presence of small amounts of large cations, e.g.strontium. A different mechanism must be active in most cases whereCaCOr crystall izes as aragonite in nature (see e.g. Lippmann, 1960 andHeinrich and Levinson, 1961, p. Aa\.

Acr<NowTBuGMENTS

While a postdoctoral fellow at the University of I l l inois in 1954-55 theauthor was looking for a suitable region for a geological f ield-trip duringChristmas vacation. Dr. R. E. Grim suggested a visit to the MagnetCove area, Arkansas, and its barite mine. Mr. O. J. Benston kindly drewattention to the new barium-calcium carbonate. The optical study, thechemical analysis, and most of the powder diffraction work were donewhile on the staff of Mineralogisch-petrographisches Institut der Uni-versitdt, G<ittingen. For the single-crystal work Prof. J. Zernann,Mineralogisch-kristallographisches Institut, Gcittingen, kindly madeavailable the facil i t ies of his laboratory. His hospitality and advice aregratefully acknowledged. Dr. F. J. Eckhardt, Bundesanstalt f i ir Boden-forschung, Hannover, prepared additional r-ray spectrometer traces.I{y wife went to the trouble of typing the manuscript in a languageforeign to her.

RBnotnNcos

Ar,u, KNur F. (1958), The crystal structure of barytocalcite BaCa(COa):. Arki,o Mineral,.Geol . , 2,399-410.

Fnvrr,uwo, Vnnur C., Jn. .rxo Dnlw F. Hor,snooK (1950), Titanium ore deposits of HotSprings County, Arkansas. Ark. Res. Dewl,. Comm. Dilis. Geol., BulL 16.

Golosurtn, J. R. aro D. L. Gnel (1958), Relation between lattice constants and composi-tion of the Ca-Mg carbonates. Am. Mineral.,43, 8rt-101.

Gosswnn, B. nxo F. Musscrwc (1930), Uber Alstonit und Milarit. Ein Beitrag zur Kennt-nis komplex gebauter Kristalle. Cent. Mi.neral,., A 1930, 220-238.

598 FRIEDRICH LIPPMANN

Gner, D. L. eNo W. F. Bnlor,rv (1962),The crystal structure of huntite MgaCa(COa)r.A cta Cryst., 15, 238-242.

Hnrwnrcn, E. Wu. aNn A. A. LnvrNsoN (1961), Carbonatic niobium-rare earth deposits,Ravalii County, Montana. Am. Mineratr.,46, 142+-1447.

Lrru.LttN, F. (1960), Versuche zur Aufkliirung der Bildungsbedingungen von Calcit undAragonit. Fort. Mi,neral., 38, 15G161.

-- (1961A'), Uber ein Barium-Calciumkarbonat aus Hot Springs County, Arkansas(abs.). F ort. IIinerd., 39, 81.

- (19618), Benstonit, CazBa6(COa)3, ein neues Mineral. Di,e Natu,rwissenschaJten,48, 550-551.

Mnosr, M. 8., E. C. T. Cueo, J. J. Fermv AND C. MrLroN (1961), Norsethite, BaMg(COr)r, a new mineral from the Green River Formation, Wyoming. Am. Mineral.,46,42H29.

PAr,Acrre, C., H. Brnu.tll, exo C. Fnorropr, (1951), Dana's System of Mineralogy Vol.II(7th): New York, John Wiley & Sons, New York.

P.tnns, Bnvln AND G. C. Bnannrn (1933), A barite deposit in Hot Springs County,Arkansas. Arh. Geol. Swrey, InJ. Circ. l.

Scuutuer, J. H., L.W. Evars, R. J. GrNrnnn, AND K. J. Munere (1947), The sensitizedluminescence of manganese-activated calcite. Jour. Applied Physics,18r 732-739.

SweNsoN, H. E. euo R. K. Fuver (1953), Standard. r-ray powder patterns. U. S. Nat.Bw. Stand.., Ci,rc.539, 5I-54.

Tene.o,t, Jrrsuo (1952), Rhombohedral crystals of Ba-Ca and Sr-Ca double carbonates.fow. Phys. Soc. Jopon,7,432434.

-- (1953), Crystal structure of the Ba, Sr and Ca triple carbonate. fow. Phys. Soc.fapan,8, 158-164.

Manuscript received., August 9, 1961.

Note added in proof : Prof. K. H. Wedepohl, Gdttingen, kindly ran a spectrographicanalysis for lead on the same material which was used for the chemical analysis of ben-stonite. The line of Pb at 2$3 A, which should still be visible at a concentration of as lowas 5 ppm of Pb could not be detected. The benstonite therefore contains less than 5 ppmof lead. This renders more likely the explanation that barium is the second activator(besides manganese), as is perhaps also the case in bariumcyanoplatinate, where ob-viously barium and platinum act together.