Canadian Mineralogist Vol. 21, pp. 705-709(1983)

STURMANITE, A FERRICIRON, BORON ANALOGUE OF

DONALD R. PEACOR Departmentol Geologicalsciences, university of Michigan,Ann Arbor, Michigan4gl0g, u.s.A. PETE J. DUNN Departmentof Mineral sciences,smithsonian Institution, woshington,D.c. 20560,u.s.A. MARJORIE DUGGAN PhelpsDodge Corporction, Douglas, Arizona, 85607,U.S.A.

ABSTRACT INTRoDUCTIoN

. Sturmanite, Cau1Fe3r,.rAl0.lMn2t-i o.z)rz.o(So+)z.r The mineral describedherein was brought to our B(oH)4} { r.2(oH) n 2sHzci,''is ferrtc'ii6ii,' uolon_ attention by Dr. Richard V. Gainesof Pottstown, analogue 99ryqniry of euringite C%Al2(SOt3(OH)r2. Pennsylvania,who recognizedit as 26Hzo, Sturmaniteoccurs on -an6'b*i'ti' being uncommon u, fiur_ and worthy of further investigation.Preliminary teneddipyramidal crystalsat the Black Rock mine, Republic X- ray powder-diffraction and optical examination 9f 99!rh Africa. f,he probable spacegroup is pitc, a in- I I. I 6(3), c 21.7 9(9)A, Z : 2. It is bipht yellowand hasone dicatedthat the mineral is relatedto ettringite, but perfectcleavage on {l0tO}. The dinsity is 1.847g/cm3 with substantial differences. Qualitative chemical (obs.), 1.855g/cm3 (calc.).Optically stunnaniteis uniaxial testsindicated the presenceof significant ferric iron (+ ),.with co= 1,500,e = I .505,but somecrystals have a core and the near-absenceof aluminum. We therefore tharis optically(- ), with e= 1.497,c,s= 1.499. It is weakly proceededon the assumptionthat it is a newmineral dichroic with O pale green and .E paJeyellow green.The speciesrelated to ettringite; this assumption was principal X-ray-diffracrion lines [d in A (intensitg@*t)] are: 5.58(70X I I 0), ( borne out by subsequentstudies. ?.q1q9-l(1-00), 3.8e(70) I I 4),2.7 74(s0)(3M), We pleasure 2.582(60)Ql6),2.215(50)e26, 320). Sturmanite is namea take in namingthis new mineralstur- manitei\ honor in honor of B. Darko Sturman,mineralogist at the Royal of B. Darko Sturmanof the Depart- Ontario Museum. ment of Mineralogy and Geology, Royal Ontario Museum, Toronto. His contributionsto systematic Keywords: sturmanite, ettringite group, new mineral mineralogy, and in particular, his studies of species,sulfate, Republic of South Africa. phosphateminerals from the Yukon, togetherwith his refinement of methods used in optical SonaN4atn_E mineralogy,reflect his devotionto mineralogy.The + mineral and the name were approvedby the Com- , .-Ll - jry*ggr_9, 9%{n.' ",.5AIo.3Mn2 o.r;rz.o(So+)2.: missionon New Mineral Names,I. M. A., prior to B(9^F)r),.r(oH) p.25H2o, eli uri' analogii; J ?iiriqul'.r publication. Holotype material is preservedat the bori fbre.de I' ettringite, C%Al2(SOr3(O il ta rr.ZAUrO. Smithsonian Institution under catalogue sturmanite sepr6sente en cristaux dipyramidiix apiatis su, number I'hdmatite et la barite d la mine Black Rock, r6publique 148261.Cotype and metatype specimensare pre- de_lAfri-quedu Sud. L"egroupe spatial probable ist p3ic; servedboth in the SmithsonianInstitution and the a I l. l6(3), c 21.79(9)4,Z = 2. Le min6ralesr iaune clair Royal Ontario Museum. possbde e1 un clivageparfait selon{ I0l0}. La densit6mesu- r6e est 1.847(obs), 1.855(calc.). La siurmanite esr oDri_ quementuni:xe (+),

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/21/4/705/3445963/705.pdf by guest on 25 September 2021 706 THE CANADIAN MINERALOGIST T,:"

ffi

Frc. l. Scanningelectron photomicrograph of a sturmanitecrystal (80x)

courtesyof Dr. E. A. J. Burke of the Vrije Univer- sity in Amsterdam, The Netherlands;a collector had obtainedthese in the Republicof South Africa. On thesespecimens, sturmanite crystals of identical habit to those on the holotype specimenencrust druse barite which, in turn, encrustsmassive hematite. Barite is also found as inclusionsin thesespecimens of sturmanite. Also acquired from Dr. Burke are brown opaque pseudomorphs after sturmanite. Theseare composedpredominantly of glpsum, with remnant sturmaniteinferred to be presentfrom weak X-ray-diffraction lines attributableto an ettringite- FIc. 2. Clinographicprojection of a sturmanitecrystal. group mineral. Thesepseudomorphous crystals en- Thedominant form is {1014},and the less prominent crust massivebarite and mimic sturmanite mor- formis {1124}. phologicallyin all but one detail, the absenceof the smallerdipyramid. On all observedspecimens, stur- manite is the last phaseto form. AssociatedMangane$e Mines of South Africa, Ltd. Details of the mineralogyof the minesin the Kalahari MoRPHoLocICAL.PHYSIcAL, AND manganesefield have beensummarized by Wilson Oprtcar- DEScRIPTIoN & Dunn (1978),who provided abundantreferences to the geology of the area. Sturmanite occurs as hexagonal dipyramidal The holotypespecimen consists of yelloweuhedral crystals,tabular on {0001}, with a width-to-height sturmanite crystals encrusting black, opaque ratio of approximately 3:1. Figure I is a SEM hematiteore. Barite is abundantas inclusions in the photomicrograph of a tlpical sturmanitecrystal, and sturmanite crystals; sturmanite with especiallyabun- Figure 2 is a projection ofan idealizedcrystal. The dant barite inclusions is turbid. Most observed crystalsare composedof dipyramids;the dominant material is transparent. one is { l0T4}, and { I 124}is presentas smalltrun- We obtained additional specimensthrough the cations.Both forms haveequivalent vitreous luster,

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/21/4/705/3445963/705.pdf by guest on 25 September 2021 STURMANITE. ANALOGUE OF ETTRINGITE 707

There is no epitaxy with associatedminerals. Stur- TABLE1. X-RAYPOWDER-DIFFRACTION DATAFOR STURMANITE manite doesexhibit parallel growth, with successive hkt !L(calc. ) {(obs.) r/Io individuals stackedparallel to {0001}. Maximum observedsize of the crystalsis 2.0 mm. All specimens 100 v. oo 9.67 100 It0 c.3u q(n 70 seenconsist of euhedralcrystals. LL2 4.97 4.97 15 Sturmanitehas one perfect ,parallel to 104 4.75 4.75 20 114 { 1010}and easilyproduced. Sturmanite is exceeding- 3.90 3.89 70 Iy brittle. The is light yellow. The luster is 204 ? AIE 3.6L2 20 vitreouson cleavagesurfaces and vitreousto slight- 2L2 3.463 3.U7 20 ly greasyon fracture 300 3.222 3.223 20 surfaces.The Mohs hardness 304 2.773 ?.774 50 is approximately2y2. The density, measuredwith 008 2.724 2.72s I heavyliquids and pycnometer,is 1.847g,/cm3, com- pared 216 2,575 2,582 60 with the calculated value of 1.855 g/cm3, 224 2.483 2.485 determinedusing the formula with 25H,O and unit- 314 2.405 2.404 cell parameters.Sturmanite does not zz6 2.212 2.zLs 50 fluorescein 320 2.2L7 ultraviolet radiation, and there is no cathodoluminescenceunder excitation by the 316 2.t57 ?.L6L 40 electron-microprobebeam. 410 2.109 2.110 J 324 a.ua.+ 2.053 10 Sturmanite is bright yellow in fresh unaltered 334 1.760 1.759 z material. Optically, sturmaniteis uniaxial ( + with 424 r.732 I.732 5 ), L.L.I2 L.727 indicesof refractionco 1.500 and e 1.505.It is weak- ly dichroic with O pale greenand E'pale yellowish green;absorption E > O. Somesturmanite crystals 418 1.668 1.668 15 - 600 1.611 L.6I2 5 are optically zoned; a uniaxial ( ) core has indices 1.578 2 of refractiona 1.499and e 1.497.Absorption and 1,.526 5 dichroismin thesesectors are similar to thoseof the r..301 2 I.261 1 optically positive material. Calculation of the Gladstone-Dalerelationship using the constantsof Mandarino (1976)yields a valueK" of 0.275for the chemicalcomposition, compared with a valueKo of ettringite commonly showsdiffraction relations con- 0.272for physicalproperties. These provide excellent sistentwith spacegroup P6r/mmc, but that this is agreementusing the compatibility relationshippro- causedby eithertwinning or disorder.As our obser- posedby Mandarino (1979). vations for sturmanite are compatible with space groupPQ/mmc (and P62c or P6rmc), and because X-Ray CnysrallocnepHy sturmanite is isostructuralwith ettringite, we con- clude that the probable space-groupfor sturmanite Single crystals studied using Weissenbergand is P31c; crystalsare either twinned or disorderedto precessiontechniques show that stqrmaniteis hex- producethe apparenthigher symmetry,by analogy agonal, a ll.16(3), c 21J9(9) A. These final with ettringite. parameterswere obtained by least-squaresrefine- ment of powder-diffraction data obtained from a CnsN4rsrny I 14.6-mm-diameterGandolfi-camera photograph of a polycrystalline specimenwith Si as an internal An emission-spectrographanalysis of sturmanite standard,and CuKa X radiation. The powder data by D. C. arc showedvery strong lines of calcium, are listed in Table l. strontium and barium, strong lines of boron and The diffraction data are similar to those of aluminum, and linesof moderateintensity for iron, membersof the ettringitegroup. In particular, there manganese and silicon. Qualitative electron- is a very pronounced subcell having parameters microprobe scansalso showedsignificant Ca, Fe and A = o and C: c/2; i.e." all reflections having S, with minor amountsof Al and Mn. Fluorine and l:2n + I arevery weak. Extinctionsare present for phosphoruswere found to be presentin amountsless reflectionshhl, I :2n + l, as consistentwith a glide than 0.1 and 1.0 fi.90, respectively. plane. As all reflections having I :2n + I are ex- Sturmanite is readily soluble in cold 3N tremely weak (most are not observableeven on hydrochloric acid with the evolution of CO,. photographs obtained with long exposures),the resultingin an insolubleresidue of 25 wt.7o, whifrL assignmentof the glide planeis more problematical wassubsequently identified asbarite. No significant than is the usualcase. Moore & Taylor (1970)deter- barium or strontium was found in the solublefrac- mined the of ettringite and conclud- tion. Optical examinationalso revealed,in addition ed that the true space-groupis P3lc. Theynoted that to barite, the presenceof fluid inclusionscontain-

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/21/4/705/3445963/705.pdf by guest on 25 September 2021 708 THE CANADIAN MINERALOGIST

TABLE2. CHEITIICALANALYTICAL DATA FOR STURI4ANITE

1. 2. 3. 4. e Cao 24.8 22.s 24; ,u1 ,u* Fe203 8.09 7.45 7.22 7.18 7.62 8.84 9.11 MnO 0.90 0.87 1.30 t.lb 41203 1.0 0.8 1.13 1.08 SO: 13.7 L2.2 74.02 (16' 91* (1A.21* Bzog l.6a a.o (:.s)* (3.21* J.Id Hzo 46.7 45.83

7. 1974ug (lmpure),includes soluble BaSo, and soluble CaCo? and llquid inclusions. 2. 4360ug (lmpure),lncludes soluble BaS0; and soluble CaC0i and llquld inclusions. 3. 2005ug (lmpure),includes soluble BaS0l and soluble caco; and liquid inclusions. 1. 9?99u9 (impure),includes_solub]e BaSo[ and soluble CaCo: and Ilquld inclusions. 5. 5045ug solublematerial (24.4 percentTnsoluble). 6. 142ug optically purematerial. 7. 229vg optically purematerial, Penfleldnethodr. 8. rheor;iltal comio'sitionror cai.o(re3f uA]0.3Mn26.z)(s04)2.3{B(oH)4}t.z (0H)tz.o. 25.0 Hzo. *Va'luesin parenthesesare normalizedon the basis of preclse Fef determinations.

ing radial, fibrous aluminum sulfate. Calcite was also is tentative pending a crystal-structuredetermination. a solid contaminant.Optically pure materialdid not The paucity of materialprecluded determinations of evolve CO2when dissolvedin acid, indicating that the oxidation stateof Mn. Although the presenceof the CO, can be ascribedto calcite. Fe3* and Al suggeststhat the Mn might be present The chemical analysis of cations was done by as Mn3+, the absenceof the typical color absorp- atomic-absorptionspectroscopy with a precisionof tion of Mn3* and the effectsof Jahn-Teller distor- t 2Voof the amount present.Sulfate was determined tions suggestthat Mn3* is not present.Inasmuch as indirectly usingamino perimidine;the precisionwas jouravskite, an isostructural phase, has Mna' t lOVoof the amount present.Boron wasdetermin- repofied, manganesein this oxidation statemight be ed spectrophotometricallyusing carminic acid. The a possibility in sturmanite, but that is speculative. precision of the boron analysis is approximately We have tentatively assignedthe Mn to Mn2*. + 590 of the amount present.Water was determin' ed using the Penfield method. DtscusstoN The fact that most of the chemicalanalyses were carried out with impure material causedsome dif- The data for sturmanite indicate that it is a ficulty. Accordingly, 142 microgramsof optically memberof the ettringite group, which includes,for pure material were analyzed for iron, calcium' example,ettringite caAlr(sor3(oH) t2.26H2Oand manganeseand aluminum. Water was determined bentorite Ca6(Cr,Al)2(SOr3(OH)r2.26H2O.The lat- on another 229 micrograms of optically pure ter was recentlydiscovered in the "MottledZone" material. Becauseof the limited availability of pure in Israel(Gross 1980). material, all other analyseswere performed on Taylor (1973)reviewed the crystalchemistry of the material with some impurities as describedabove. ettringitegroup. He notedthat the structureconsists Sulfur and boron resultsprovide problems inasmuch of columns (parallel to c) of Al octahedrally co- asboth elementsmight be presentin the fluid inclu- ordinatedby hydroxyl ions, alternatingwith groups sions. Becauseof the scarcity of crystalsthat lack of 3 Ca ions co-ordinatedby four hydroxyl and four fluid inclusions, we cannot give more accurate H2O. These columns have the composition analyticaldata at this time. The valuesfor Ca in col- Ca6{Al(OH)u}z.24HzO, The eight-co-ordinatedCa umns I through 5 (Table 2) are high owing to solu- mai Ue replacedby ions such as Pb2*, and oc- ble inclusions of calcite. MeasuredSO3 and BrO, tahedrallyco-ordinated Al replacedby Fe, Mn, elc. valuesin columns3, 4 and 5 (Table2) are "as deter- Channels between the columns are occupied by mined" values,coupled (in parentheses)with values anionic groups such as (SOo;z-and (CO3)2- or by corrected by normalization, based on the Fe2O3 2H2O. There are four anion-sitesper 6 Ca. These values of 8.84V0by weight, obtained on optically may be occupiedby combinationsof variousanionic pure material.Because these S and B valuesare sub- groups or H2O that must sum to four (or less)per ject to a greaterstandard error than the valuesob- 6 eight-foldco-ordinated ions suchas Ca. The repeat tained for other elements,interpretation of thesedata distancealong a column is C : c/2, the prominent

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/21/4/705/3445963/705.pdf by guest on 25 September 2021 STURMANITE, ANALOGUE OF ETTRINGITE 709

translationof the substructure,with the superstruc- tringite group. The ideal26HrO are composedof 24 ture being due to ordering of anionic groups in the within the columnsof the structure,plus two from sitesbetween columns. the channelsbetween the columns. We proposean The chemical formula for sturmanite was excessof SOnand B(OH)4 in those channels.This calculatedusing the following weight percentages, is compensatedfor, in part, by a deficienryof HrO. as derivedfrom the chemicalcompositions in Table In addition, we recognizethe difficulty of HrO 2: CaO 25.6,Fe2O'9.94, Al2o3 1.13,Mno 1.30, determinationson such a hiehly hydrated mineral B2O33.2, SO314.2, H2O 46.7,sum : l0l.OVo. and note that part of the variance in HrO can be This analysisyields, on the basis of 6 Ca atoms: *,.rAlo.rMn2* due to analytical error. cau.o(Fe3 0.2)"r.0(so4)2.3{ B(oH)a},., (oH)D.0.25.7H2O. ACKNOwLEDGEMENTS As noted previously, the analytical values for S and B are greater subject to a error than those for The authors are indebted to Dr. S. A. Williams other elements.We were able to obtain a Drecise for the separationof optically pure materialand op- value for Fe2O3on optically pure material tical analysis of liquid inclusions and (specimen barite im- #6, Table 2). Accordingly, we normaliz- purities. We thank Lance Talley for emission- ed the S and B valuesof specimens3, 4 and 5 based spectrographanalyses and Dr. E. A. J. Burke for on values required to correct the low Fe valuesof supplying additional specimensof sturmanite. We specimens 3, 4 and 5 to the precisevalue of com- are grateful to Dr. J. Jambor and the referees position for 6. We have chosenthe lower normalized their reviewsof this manuscript. values (14.2V0SO3 and 3.2V0 B2O) for use in calculation of the formula, in part becausethe higher REFERENCES values would lead to an excessivelyhigh wt.Vo summation. Gnoss, S. (1980):Bentorite, a new mineral from the Sturmanitecan thereforebe characterizedas the Hatrurim area, west ofthe Dead Sea, Israel. lsrael Fe3* and B analogue of ettringite, with the oc- J. Earth Sci. 29,81-84. tahedrally co-ordinatedsites in the columns occupied primarily by Fe:+ in the type material, and also by HunLnur, C. S., Jn. & Bauv, J. L. (1960).Ettringite Al and Mn. Although substitutionof a boron anionic from Franklin, New Jersey. Amer. Mineral, 45, group for (SOa)2-is uncommon, we note that B ll37-tt43. was describedin an ettringitelike mineral from Maroenrro, (1976): Franklin, New Jersey,by Hurlbut (1960). J.A. The Gladstone-Dale rela- & Baum tionship, The I. Derivation of new constants. Cdr?. Franklin material is presently under re- Mineral. 14,498-502. investigation;it has a S:B ratio very similar to thar of sturmanite and is likely the Al-analogue of (1979):The Gladstone-Dale relationship. IIl. sturmanite. Somegeneral applications. Con. Mineral. l7 ,7l-76. An idealizedformula for sturmaniteis tentative, in part owing to someambiguity in the number of Moonr, A.F. & Tavlon, H.F.W. (1970): Crystal struc- S and B atoms. The formula is provisionally given ture of ettringite. Acta Cryst. B,26, 386-393. *,A1, * as Cau(Fe3 Mn2 )2(Sor2.3{B(oH)4} r.z(oH)r.z. Tavlon, H. F. W. (1973):Crystal 25H2O. The assignmentof B to a B(OH). group strucruresof some double hydroxide minerals. Mineral. Mag. 39, i.e., to tetrahedral co-ordination, is not basedon 377-389. direct evidence.This anionic group was specified because:(l) it is consistentwith overall charge- WrLsor.r,W. E. & DuNN, P. J. (1978): Famous mineral balancerequirements, (2) it is in agreementwith the localities: the Kalahari manganese freld,.Mineral. total H2O determined,and (3) it is reasonableby Rec. 9. 137-153. analogy with other boron minerals. The total HrO is lessthan the26 in the idealizedformula, consis- Received October 28, 1982, revised manuscript accepted tent with the crystal-structurerelations of the et- March 6, 1983.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/21/4/705/3445963/705.pdf by guest on 25 September 2021