881
The Canad,ian Mineralo gist Vol. 34, pp. 881-892(1996)
PHASETRANSITIONS IN THESERIES BORACITE- TREMBATHITE- GONGOLITE: PHASERELAflONS
PETERC. BURNSTar.ll MICHAEL A. CARPENTER
Department of Earth Sciences, University of Canbridge, Downing Steet, Cambridge CB2 3EQ, U.K.
Ansrnq.cr
The Sussex,New Brunswick,marine-evaporile-hosted borate deposits contain boracite-group minerals of the solid-solution seriesMgrBrOtrCl - FerBrO'.CI. Samplesof boracite,txembathite and congolite have beencharacterized by electron-probe microanalysisand by high-temperatureoptical examinationand X-ray powder diffraction. At 25oC,the orthorhombicboracite structure(PcA) is stablefor compositionsfrom MgrBrO,rCl to (Mgl.eFet.l)B?Ot3cl,and the rhombohedralcongolite structure(jR3c) occurs for compositionsranging from (Mg1.eFe1.1)B7O,uClto FerBrO,rCl. At high temperatures,all specimens examinedhave the cubic boracitestructure (F43c). Cooling resultsin a frst-order phasetransition to the orthorhombicstruc- ture, and specimenswith more than 36 mol.VoFerBrO,rCl undergoa further first-order phasetransition to the rhombohedral structure.The boundariesbetween the phaseswith cubic, orthorhombicand rhombohedralstructures are linear in composition- temperaturespace. There is a markeddiscontinuity in the unit-cell volume of specimensrich in Mg at the phasetransition from the cubic to the orthorhombicstructure; the positive volume strain is greatestin the Mg-rich samples.The high-temperature X-ray diffraction data show that the cubic-to-orthorhombicphase transition is frst order in samplesrich in Mg, but increasing Fe contentresults in a trend towaxdtricritical or second-ordercharacter. Higher Fe contentsare associatedwith a decreasein the absolutevalue of the volume strain due to the phasetransition. Although the symmetry-breakingstrain due to the phase transition is very small, it is revealed by the variation in peak width as a function of temperaturein the X-ray powder diffractograms.
Keywords:boracite, trembathite, congolite, phase transition, phase relations, solid solution,mineral physics.
SoNttu.a.rns
l€s d6p6tsde boratede Sussex,au Nouveau-Brunswick,d'origine dvaporitiquemarine, contiennent des mindraux du groupe de la boracite,membres de la solutionsolide Mg3B7O,3Cl - FerBrOl3Cl.Nous avons caractdris6 les 6chantillonsde boracite, trembathiteet congolitepar analysei la microsonde6lectronique, par ex:rmendes propri6t6s optiques d tempdrature61ev6e, et par diffractionX. A25"C, la structurede la boraciteorthorhombique (Pca21) s'applique aux compositionsdans I'intervalle MgrBtO'rCl d (Mgr.eFer.r)B7Or,Cl,et la structurerhomboddrique de la congolite(R3c) s'appliqueaux compositionsdans I'intervalleentre (Mgr.nFer.r)B7ol3cl i FqBrO,rCl. A temp6rature6lev6e, tous les sp6cimens6tudi6s possbdent la strucfure de la boracitecubique (F43c). Un refroidissementcause une inversionde premier ordre i la structureorthorhombique; de plus, tout 6chantilloncontenant plus de 36To de Fe3BrO,,Cl (base molaire) subit une deuxidmetransition, a h structure rhombo6drique.les limites deschamps de stabilit6 desphases ayant les structurescubique, orthorhombique et rhombo6drique sont lin6aires dans un diagrammede composition versus temperature.Nous ddcelonsune discontinuit6importante dans le volume de la maille 6l6mentairedans le cas d'6chantillons riches en Mg en passantde la structurecubique h la structure orthorhombique;la distorsiondu r6seauassoci6e h la transitionest plus importantedans les dchantillonsles plus riches en Mg. Les donndesde diffraction X obtenuese rcmpdrature61ev6e montrent que cettetransition a un caractdrede premier ordre dans le cas d'6chantillonsriches en Mg, mais avecune augmentationen Fe, son caractdredevient tricritique ou de deuxibmeordre. lrs teneursplus 6lev6esen Fe mdnentd une diminution du degr6 de distorsion volumique due i l'inversion. La distorsion associ6ei l'inversion requisepour r6duire la sym6trieest trOsfaible; malgr6ceci, la largeurdes pics de diffraction en fonction de la temp6ratffeen est un reflet sensible. (Traduit par la R6daction)
Mots-clds:boracite, trembathite, congolite, ffansition de phase,relations de phases,solution solide,physique min6rale.
I Presentaddress: Department of Geology,University of lllinois at Urbana-Champaign,245Natural History Building, 1301 WestGreen Street, Urbana. Illinois 61801.U.S.A. 882 THE CANADIAN MINERALOGIST
INTRoDUcTIoN specimens(due to these phasetransitions), and will illustrate how the microstructurerelates to the thermal Boracite-type phases have the general formula history of boratedeposits. M3B,OBX, whereM is a divalent cation (Mg, Cr, Mn, Most of the boracite-groupmineral specimens Fe, Co, Ni, Cu, Zn, Cd), and X is a halogen atom examined in this study are from the Penobsquis (Cl, Br, I). These phases have been synthetically evaporitedeposit, located in Sussex,New Brunswick. preparedand extensivelystudied, owing to the ferro- Since their discovery two decadesago, the marine- elastic, fenoelectric and magnetic properties of the evaporite-hostedborate deposits at Sussex have crystals Q.lelmes 1974). Five of the boracite-type received considerableattention (Roulston & Waugh phaseshave been describedas minerals (Table 1); all 1981, Rachfin et al. 1,986,Mandarino et al. 1990, have Cl as the halogen,and four containMg and Fe as Bums et al. 1992" Roberts et al. 1993. Gice et al. the divalent cation. Crystals of natural boracite-group 1994).The Mississippian-agedeposits occur in the mineralsusually containcomplex twinning anddisplay Windsor Group of the Moncton sub-basin,a part of anomalous optical properties, although some well- the northeasterly trending Fundy basin. Borate formed specimensshow excellent large domains. minerals are primarily located in the Upper Halite Schmid& Tippmann(1978) provided an interesting Memberfnomenclature of Roulston& Waugh(1983); historical overview of early studiesof boracite-group this is refened to as the Middle Halite Member by minerals. Roulston& Waugh (1981)1.Details of the geology The phasetransitions in boracite-typephases hpvp of the evaporite depositsmay be found in Roulston received a great deal oJ attention over the past three & Waugh (1983).In additionto a suite of a dozen decadesbecause of possible applications in the or more previously recognized borate mineral material sciences.For example,boracite-type phases species, mineralogical studies of these deposits have been considered as possible components for have led to the descriptions of the boracite-group optical page composers,optical storage,and passive mineral trembathite (Burns el al. 1992) and t}te display devices becausethey are ferroelecrics that CarBruO3a(OH)%C14.l3H2O polymorphs pringleite permit optical contrast upon switching (Schmid & and ruitenbergite (Roberts et al. 1993), whose Tippmann 1978). A necessaryrequirement for using structuresbear similarities to those of aluminosilicate boracite-typephases in this contextis the development zeolites(Gice et aI. 1994). of a composition having a very small shear angle The Zechsteinsalt depositsof Germanyalso contain in order to prevent cracking and various other boracite-gtoupminerals of a range of compositions. detrimental effects outlined by Schmid & Tippmann Heide& Vijlksch (1979)and Heide et aL (1980)have (1e78). examinedthe boratesin these deposits;their studies This study concernsphase relations in the series included chemical analyses, studies of crystal Mg3BrOtrCl - FerBTOrrCl.The seriesincludes the morphology by scanningelectron microscopy,X-ray minerals boracite [Mg3B7O13Cl], trembatlite diffraction, and differential thermal analyses. The [(Mg,Fe).BrO,rCl]and congolite[(Fe,Mg)rBrO13Cl]. Zechsteindeposits contain both well-formed pseudo- The structure of boracite is orthorhombic at room cubic crystals and fibrous intergrowths of boracite- temperature,whereas the structure of congolite is group minerals.Heide & Vdlksch (1979) indicated rhombohedral.Thus a solid-solution series between that the pseudocubiccrystals may have formed at Mg3B7Ol3Cland FqB7O13Clmust involve a phase several different times, and that intergrowths of transition. The purposeof this work is to characterize mimetic twins are absent. This absence of twins the phaserelations in this system,both as a function of indicatesthat the crystalsgrew in the stability field of Mg:Fe ratio and of temperature.In later contributions, the orthorhombicor rhombohedralstructure" and that a we will examine the microstructure in natural phasetransition has not occurred.Heide et al. (1980) reported chemical compositionsfor 21 specimensof boracite-groupminerals from the Zechsteindeposits. Crystal compositions range from end-member Mg3B7Ol3Clto (Fe2.54Mno.rnMgs.2)B7O13Cl.
TABI.E 1. CELL DIMENSIONS OF BORACITE-GROUP1\4INENAT6 STRUCruRE AND PHASE TRANSITIONS Spae Grup o (A) o tAl c (b R€€ boracite-typephases boncit€ MgdrOn0l Pa2t 8.677(6) 8.668(8) 12.09(1) 1 The crystal structuresof all have cubic symmetry, space group F43c, at ht$ qi€ito (FeJVIglBzOrCl Po2: d.od d.oo \2.17 2 tr@bathite (Mg,Fe)rBrOuq A& 8.67q2) 20.99(1) 3 temperature.The structureof cubic boracite (Ito et al, mngoliie (Fe,Mg)gBrOrClE& 8.62.211) 21.064{6) 4 1951,Sueno et aL 1973)consists of a frameworkof chmbsite MnsB0rsgl Pw2t 8.68(1) 8.68(1) 12.26(1) 6 comer-sharing BO4 tetrahedra,with the metal and halogen atoms located within cavities in the borate MeM:(\)Bw (1996);(2)Kiiba& Schuc.ke(1966); (8)BlJruetal (1992); (4) Wentlliag et @l (19?2); (6) Hon@ & Beck (1962). framework(Frg. l). Using the descriptorof Burns el a/. TRANSITIONSIN BORACITE-GROIJPPHASES 883
structuresis A6n: [0]4n{4n>k3n>1L. T\e Pca2 1 and R3c structurescontain three and one symmetry- distinct metal sites, respectively. In each case, the tli coordination geometry of the cation is significantly distortedfrom the arrangementof the cubic structure. The phase transition from the cubic boracite structure,space group F43c, to the orthorhombic structure,space group PcaZ1,'ls'an improper ferro- elastic phase-transition(Dvol6k & Petzelt L97l) that ill involves a doubling of the volume of the primitive unit-cell (Ito et al. 1951). The orthorhombicand rhombohedral unit-cells are obtained from the cubic unit-cell via the transformation matrices lYz,Vz, 0, 42, L/2,0, 0, 0, ll and Ir/2,0, 1/zo- Yz,Vz, 0, -1, -1, 1],respectively. Upon heating, synthetic Mg3B7O13Clundergoes a singlephase-transition from the orthorhombicstrucfure (Pca2) to the cubic structure (F43c) at -264"C (Schmid& Tippmann1978). When heated,synthetic FqB7O13Clundergoes a seriesof first-orderphase- Frc. l. The cubic (Sueno transitions from the rhombohedral structure (R3c;, boracite struciue er al. 1,973) -255"C, projecteddown [001]. BO4 tetrahedraare shadedwith frst to a monoclinicstructure (Pc) at thento crosses,Mg atoms as shown as circles shadedwith a an orthorhombic structure (Pca2) at -770oC, and. herring-bonepattern, and Cl atoms are shown as open finally to a cubicstructure (F43c) at -336C (Schmid circles.The cubicunit-cell is outlined. & Tippmann1978).
E>ennnrnsNTAL
Sampledescription
The specimensstudied, all but one from the Potash (1995),the tundamentalbuilding block (FBB) for this Company of Saskatchewanmine workings, are structureis an:tQlnlnlnlnl, indicatingthat a central currently housedat the CanadianMuseum of Nature oxygen atom is connectedto four Bo, tetrahedra.The (CMN) in Ottawa,Ontario. Each sample suite contains structureof cubic boraciteis the only structureknown one or more sectionsof diamond-drill core from the to containan oxygenatom [O(l) usingthe notationof Upper Halite Member. In addition to halite, eachdrill Steno et al. L9731that is bondedto four boron atoms. core containsan assemblageof borateminerals, which The O(1) atom is located on a site with point includes hilgardite, hydroboracite,and the boracite- symmetry 23, and in cubic boracite, e-achof the four group minerals. Pieces of drill core were dissolved B(3)-O(1) bondJengthsis 1.693(5)A (Suenoer aL in warm water, and boracite-group minerals were 1973), a value considerablylonger than
TABLE 2. EUECIRON.IVIICROPROBEDATA FOR BOMCNE.GROI'P MINERAI"9*
sPoilt8 L6 A 3063035 FeO ,6l{.36) 26,33(66) 23.89(66) 37.01(1.69)36.800.24) 12.67(42) 12.44(8) A U) M4O 0.26(4) 0.21\4) 0.38(5) 1.0(x2) 0.84(12) 0.89(?) 0.90(6) 0.09(3) MsO 13.62(40) 18.1(41,) 14.9r(36) 4.4il1.2r5) 6.02(95) 22.86{U) 22.Lq$) 30.62(48) cl 7.83(18) ?.62(13) 8.08(9) 7.49(8) 7.48(8) 8.30(10) 8.47(6) 815(40) B2& 66.42 66.43 67.67 6L.94 62.11 60.26 69,71 66.21 cl=o -L.77 -1,72 -1.82 -1.69 -1.8? .1.91 -1.86 Total 101.80 102.01 108.08 100.19 100.06 102.60 101.73 106.39 Fd. 1.66(3) 1.68(4) 1.41(3) 2.4%13) 2.36(10) 0.?1(1) 0,70t2) 0.112) Iyhs" 0.00(2) 0.016(3) 0.023(3) 0.07(1) o.o8(1) o.o6q3) o.o5{3) o.ooq2) Ms 1.45(8) 1.4q4) 1.67(3) 0.62\t4) 0.68(11) 22e! 2.2.4,(2) 2.83{2) cl 0.95@) 0.93(2) 0.96(1) 0.99(1) 0.99(1) 0.95(1) 0.97(1) 0.87(6)
t BuOr elculated 6oa stoichiometr$ fomula elolatioa mue Fe + Mn + Mg = 3
EI e c t r on-mic r opro b e analys i s slit, a 1o receiving slit, and a diffracted-beam monochromator. Preliminary scans showed the Single crystals were cut, mounted in epoxy, patterns to be featurelessbelow 14"2Q; data wete polished, and coated with carbon for electron- collectedover tle range 14 - 100'2e with a step microprobe analysis. Compositionswere determined interval of 0.02'20 and a count time of 60 s per using a CamecaSX50 probe operatedin wavelength- step. The symmetry of each sample (Table 3) was dispersionmode. The accelerationvoltage was 20 kV, determinedby comparisonof the powder patternsto and the beam current was 15 nA; the sarnpleswere thosegiven by Burns(1995). found to be stable under these conditions. The following standardswere used: MgO (IvIg), Fe metal High-ternperatureX-ray powder dffiaction @e),NaCl (Cl), Mn metal (Mn) and wollastonite(Ca). Multiple points were analyzed for each sample; Samples for high+emperatureX-ray powder- average values for each sample, along with the diffraction experimentswere ground in an agatemortar correspondingstandard deviation, are reported in for severalminutes. Silicon was addedas an internal Table 2. The concentrationof boron was calculated standard.The sampleswere smearedonto a heating from stoichiometry. device that consisted of a platinum ship pulled Significant Fe - Mg zonation is apparentin back- mechanically onto a ceramic plate. The sample scatleredimages, and in the relatively high standard thickness was roughly 0.2 mm, and the temperature deviations of the averageanalysis for each specimen was monitored using a platinum - l3%o rhodium (Iable 2). The large (001)cubicgrowth-sectors contain thermocouplewelded to the undersideof the platinum concentric zones parallel to the growth faces that strip. The temperature was calibrated against the become progressively more iron-rich moving from transition points of various standardsmounted in the core to the rim. In addition, (lll)cubic growth- the sameconfiguration; the uncertaintyof the tempera- sectorsare in somecases present, and are enrichedin ture scale is +10'C. Diffraction data were collected either Mg or Fe relative to the adjacent(001).u6,. growth-sectors.These sector zones also contain concentriczoning parallel to the (1I l)cub1"growth-face. Somecrystals contain a prominentFe-rich overgrowth. TABI,E 8. ROOM.TEMPERATURE SY'I4METRYAND PIIASE. Compositionsfrom small (Ill)cubic sector-zonesand TRANSINON TEMPERATUNES FOR BORACTTE.GROIIP MINERAT"S overgrowthswere excludedfrom the averagesreporled in Table 2. Minsai M Room T. RScePm% Pm2teF4k Nme Fomula SbustuF
Room-ternperatureX-ray pow der dffiaction 38-A cmgolite Ferotr{gro 89c 87 t]'Z'C 911 r 5oC S8-B congolite Fer*&Igreovluor E8c 96!10 307t0 Samplesfor X-ray powder-diffractionexperiments 38-C trobathite MgrrFeutrtaqo Bgic 60r1? 908t6 ground S&D congolite FeeMgooMro.o E& 217t7 82,0t2 were crushedand in an agatemortar for several S8-E congolite Fez:elVlgooMnoo l?& 208t7 819! 3 minutes. Silicon was added as an intemal standard. S4-A bomcite MgexFeori[lnooPco2r 287xa and smearmounts were preparedon low-background S4-B bomcite MgaFeootr4no.o Pm2r 2&3t2 lead-glass.Diffraction patterns were collected at25oC EB boncite Mg8Fe.!r Pw2t 272t6 with powder Fe3 336 an automatedSiefert X-ray diffractometer Iuso 4 with Bragg-Brentano geometry using CuKa X-radiation (20 mA, 30 kV), a fixed l-mm divergence 3&m $cbmid & Tippmm (1978) TRANSMONS IN BORACITE.GROUPPHASES 885
TABLE 4. I{IGH.IEMPERAfURE UNN.CEI'PARAMETERS AND TABI,E 6. HIGII.TEMFERIffUNE UNIT.CELL PARAMETERS AND IINIT{EITVOLUME FOREB I.INIT{ELL VOLUME T'OB S&A rec) o (A) v(Aq r("c) 4(A) y(Ar) reo @(A) v(A) r ec) @(A) v(As)
20 12.093(5) 1768.8(2) 267 12.1067(5) L774.1(2) 20 72.142/.r) 1790.3(4) n8 12.$2{2) 7794.7(8) 203 72.@42(6) 7769.0{2) 261 12.1039(6) 1778.3{2) <204 12.189(1) 1788.8(5) 2 12.168(1) 1794.{6) 97 12.0991(6) 177I.43') 266 12.1039(4) t'778.8\2) 39 12.14q1) 1789.3(6) 2 u.164(1) 1796.3(7) 12t 12.1026(6) rn2.7e) 290 12.1066(6) 7774.0{2) OY 12.149(1) 1790.6(6) 262 12.158(2) 1796.1(7) 7& 12.1041(6) r77g.Al2' 314 12.106(6) 7774.6/.9) 68 19.143(1) 1790.6(6) 26r 12.16(1) 7796.4(7) 170 12.1066(6) r774.q2) 339 12.1094{6) 77'16.7(2) 78 12.14 2) r.790.0(9) 27L u.16(2) 1796.q7) 194 12.1068(5) 1774.8io\ 368 12.1114(6) fil6.q2) 88 12.142(1) 1790.q5) 28t 12.165(2) 1796.9(8) 278 12.1073(6) 7774.7Q) 887 12.1L22(6) 1777.u2) 98 12.144r) 1?90.8(6) 290 u.16(1) 7796.2(n 2 12.1082(6) L776.1(2) 471. 12.LL48(7) 1778.1(3) 107 l?.142(1) 1790.3(6) 300 12.159(1) 1197.7(n 2A2t 12.1076(5) 1774.9(2) 436 12.1168(6) L778.7(2) LT7 t2.14L(2) 1?89.q7) 310 12.16(1) fis8.4n 7 12.1076(5) 1774.9(2) 469 12.1188(4) 1779.8{2) 126 12.143{1) 1790.(6) 319 12.168(1) 1?97.3(6) 262 72.1072(6) 1774.7(2) 483 12.120X6) 1780.6(31 12.146(1) 779r.8$) 829 u.16(1) 179?.9(6) 146 72,747(!) 1789.8(6) ' 339 12.16(1) 1798.(6) obtain€d atts coolitrg too 12.146{1) 1791.3(6) 3Gl 12.16t(1) 1799.6(?) 166 12.149(1) 1793.3(6) 387 12.16(1) 1800.6(?) 12.742\t) L79{.4O 411 u.166(1) 1800.8(6) 184 12.161(1) 1794.1(6) 4:t5 L2.16't(2) 1801.1(7) 194 12.16(2) 1798.6(8) 69 12.168(1) 1801.7(6) 20s u.16q1) 1798.6(n 483 u.1?q1) 1802.7(6) 219 19.154(1) 1798.4{6) using monochromatedCuKcr,l X-radiation (22 mA, lobtaued alter @I[g 35 kV) and a 4K-PSD INEL detector (as described by Salje et al. L993). The diffraction signals were measuredbetween 20.;o = 8 and 20* = l20o for 3 hours per temperaturesetting, and data were corrected using all ten reflections correspondingto deterrninedunit-cell parametersdecreases significantly the silicon internal standard. Unit-cell parameters with increasingFe, to the extentthat useful data could (Tables4, 5, 6) were refined using the leist-squares not be obtained for the most Fe-rich specimens. progam of Appleman & Evans (L973). The least- Therefore,data are reported for samplesF3, S4-A and squaresrefinements for sampleswith cubic symmetry S8-A only. included -35 reflections, whereasthose for samples with rhombohedral and orthorhombic symmetry Optical examination included -55 reflections. The unit cells of the rhombohedral and orthorhombic structures are Crystals were cut parallel to (001).u61"and doubly dimensionallycubic within the resolution of conven- polished to a thickness of -30 pm for optical tional X-ray powder diffraction @ums 1995). Hence, examination.Each sectionwas studiedusing polarized all unit-cell refinementswere performed using cubic light and a microscope equipped with a Linkam unit-cell constraints,and the reporied dimensionsare Scientific Instrumentsheating stage(model TH1500). given in the cubic setting. The heatingstage was calibratedagainst known phase- Owing to the lack of a diffracted-beam mono- transitions and melting points, and is accurate to chromating device, and the fluorescence of Fe in within t2'C. The purposeof the optical examination Cu X-radiation, backgroundcounts increased dramati- was to determine the temperatureof the cubic+o- cally with increasingFe contentof the samples.Owing orthorhombic and orthorhombic-to-rhombohedral to a poor peak-to-backgroundratio, the precisionof the phase transitions (Table 3). This was easily accom- plished owing to the large shangesin the birefringence at eachtransition. It is possiblethat a monoclinic phase TABI,E 6. IIIGH.TEMPEMTUNE UNIT-CELL PARAMETSRS AND is stableover a narrow interval of temperaturebetween I]NTT.CEI;L VOLUME FOB S4.A the rhombohedraland orthorhombic phases,on the T("c) o(A vfAel r("c) o(A) vtA'l basisof studiesof end-memberFerBrOr3Cl (Schmid & Tippmann 1978). The optical properties of the 20 12.1178(6) 1779.1(3) 228 12.7277(8) 1?83.6(8) 2O3 12.11@(6) Lng.d'2) 292 L2.7277(n 1783.(3) monoclinic and orthorhombicphases are very similar 89 12.1204(6) 1780.(3) 2 \2.1276(6) 1783.7(3) (and very different from those of the rhombohedral 69 12.1208(6) 1780.7(9) 262 12.12?(6) 1?83.7(9) 7A p.r2,2qi6) 1781.2{3) 261 12.129q6) 1784.3(3) phase)in the (001)"o51"section (Schmid & Tippmann 98 t2.1227(6) 1781.(3) 27r 12.1U!qD 1784.2(8) to? t2.721xn 1780.9(8) | 72.L268(n 178$.4(8) 1978).We sawno evidencefor themonoclinic phase in 1r7 L2.12.82(6)1781.(3) 290 72.1281(7) 1?83.{9) the intermediate-compositionsamples studied here. t2ts 12.724/.8') L78L,7(4) 300 r2.L2n6) 1789.(9) 136 72.123616)1781.9t8) 310 12.128:l(6) 1784.(9) Considerablechemical 2ening occurs within some 1116 12.124ryi6) 1782.1(S) 319 L2.t2U(6) 1784.1(8) 165 L2.124.6(7)L782.qg) 329 12.1306(6) 1786.(2) of the crystals (Table 2); in such cas€s,a range of 166 L2.r246(6) v82.4{.9) 348 12.1909(6) 1786.2(8) transition temperatures was observed for a given 176 L2.L284(6) 1782.7(2) 867 A.By7(7) U86.9(S) 184 12.U6q6) 1782.q3) 387 12j82,Q) 1?86.&3) crystal; the temperatureof the onsetand completionof r94 12.125{6) 1783.0(3) 4tl6 r2.r882(7) 1788.4(S) 28 12.1268(6)1788.4(8) 483 12.7420d8) L779.012) the phase transition was noted, and the temperafure 273 12.1269(6) 1788.4t9) uncertaintiesreported in Table 3 spanthe entire range 'obtai!€d aIX€r @liag of temperatureover which the transition occurred. TI{E CANADIAN MINERALOGIST
Detailed studiesof the spontaneousbirefringence as a temperature optical exarnination has permitted the function of temperaturewere not attemptedbecause the identification of the phase-transitiontemperature for nahrralcrystals contain multiple ds11nins,and detailed thesespecimens (Table 3). The phaserelations in the measurementson crystalswith poor control of multiple systemboracite - trembathite- congotte inferredfrom domainshave been shown to be proneto seriouserrors our resultsare given in Figure 2. (Schmid& Tippmann1978). All specimensstudied undergo a phase transition to the structureof cubic boracite at high temperature PHASEREI-{roNs (Fig. 2). Upon cooling,specimens of all compositions undergo an apparentlyfust-order transition from the The phasetansitions have previously been docu- cubic structure to an orthorhombic structure. mented for synthetically prepared Mg3BrOt3Cl and Specimensrich in Fe undergoan additionalfirst-order FerBrOrrCl(Schmid & Tippmann1978). The principal transition to the rhombohedral structure prior to purposeof the current study is to elucidatethe phase reachingroom temperature(Fig. 2). relationsin natural specimensthat departsignificantly The phaseboundary between the stability fields of from ideal end-membercompositions. Our X-ray the cubic and orthorhombic structure is linear in powder-diffraction studies at room temperaturehave composition-temperaturespace @ig. 2), with a slope suggestedthe space groups of specimens with of 7.1'C per l0 mol.VoFqBTOy3Cl. The boundary intermediate compositions (Table 3), and high- betweenphases with orthorhombicand rhombohedral
0 ,oo o :t E c) o- E 150 F
0.0 0.1 0.3 a.4 0.5 0.6 0.7 0.8 0.9 1.0 Mg.B,Qpl Atomicproportion Fe Fe.Bppl
Ftc. 2. Phaserelations in tle seriesboracite - trembathite- congolite. TRANSITIONSIN BORACITE-GROUPPHASES symmetry, also linear for the data collected in the boraciteis the stablephase over the range Mg3B7O13Cl presentstudy, is much steeper,with a slopeof -45"C to (Mgy.e2Fe1.ss)B7O13Cl,and the rhombohedral per 10 mol.7oFqB7O13Cl. The transitionpoint for the structure is stable from (Mg1.e2Fe1.6)BrOrrClto pureFe end-member(from Schmid& Tippmann 1978) FqB7O13Cl. At room temperature,specimens with falls below the temperatureextrapolated from inter- compositionsin the range (Mgt.qzFer.oe)BtO,rClto mediatecompositions, but this behavioris consistent (Mg1.5sFe1.5s)B?Ol3Clare trembathite, and those with a plateaueffect, as describedfor other systemsby with compositionsbetween (Mg,.5sFe1.50)B7O13Cl Salje (1995). The extrapolationto more Mg-rich and F%B7O13CIare congolite. Note that ericaite, compositionsindicates a phase-transitiontemperature defined as the orthorhombicform of @e,Mg)3B7O,3C1 of 25'C for 36 mol.VoFerBrO,rCl, corresponding to Kiihn & Schaacke195$. does not occur at room the formula (Mg1.e2Fer.u)Bzol3cl.Thus, at 25oC, temperature.
ts o E E o
0 50 '100 150 200 250 300 350 400 450 500 TemperaturefC) Fto. 3. Unit-cell volume of samplesEB [(Mg2.33Fe0.r7)B7Or3Cl,bottom], Szt-A lOIgz.zFeo.zAIno.05)B7Or3Cl,middlel andS8-A [@e,.55Mg,.rr)BrO,rC1,top]. 888 THE CANADIAN MINERALOGIST
HrcH-TEMpERATLIREX-Rev DnrnecrloN 0.0030 Variation of unit-cell volumewith temperature a 0.0025 The variation of unit-cell volume with temperature of samplesEB, S4-A andS8-A is shownin Figure3. 0.0020 In each case,the temperatureof the phasetransition, 0.0015 as determinedby optical exarnination,is indicatedby I aITOWS. 0.0010
SampleEB: T\e unit-cell volume of sampleEB, with 0.0005 composition(Mg2.sFee.17)B7Ol3Cl, shows a signifi- cant discontinuity at the transition from the cubic to 0.0000 orthorhombic structures.The transition involves an {.0005 increasein volumeof -0.l2Vo at the transitionpoint (Fig. 3). This ransition hasbeen shown to be of lrst {.0010 order(Schmid & Tippmann1978). 0.0035 0.0030 SampleS4-A: T\e unit-cell volume of sampleS4-A, with composition(Mg2.2aFe6.?tMno.06)B7O1rCl, shows 0.0025 a discontinuityat the phasetransition from the cubic to orthorhombic structures(Fig. 3). In this material, the 0.0020 volumeincrease at the transitionpoint is -0.06Vo. 0.0015 f SampleS8-A: The unit-cell volume of sampleS8-A, 0.0010 wilh composition(Mg1.45Fe1.55)B7O13Cl, does not show a significant discontinuity at either of the phase 0.0005 transitions that were detected using optical exami- nation. Owing to the poor peak-to-backgroundratio, 0.0000 which is due to Fe fluorescence.the unit-cell volumes -0,0005 could not be preciselydetermined. It is possiblethat there are discontinuitiesin the volume at thesephase -0.0010 transitions.and that the volume strain is less than the o 50 1001s0 200 250 300 350 400 450 500 precision of the measurements.The unit-cell volume Temporaturo(t) does show an apparent discontinuity with a EB volume decreaseat -200"C. A phase transition at FIc. 4. Volumestrain of samples [(Mg233Fe0.r7)B?Or3Cl, and S4-A top]. optical bottoml [(Mg2.2aFe6.7slVIne.s5)B7Or3Cl, this temperafurehas not been observedusing Broken lines and doned lines representthe best fit of study does examination,but an infrared spectroscopic the 234 @quation 4) aloLd246 (Equation 7) potentials, indicate a phasetransition near this temperaturein tlte respectively.Data collectedfor temperaturesgreater than samesample (Bums & Ca{penter1996). 252oCwere not includedin the least-squaresfit in the case The change of volume at the phase transition in of sampleEB. going from the cubic to the orthorhombic structure involvesonly a relativelysmall discontinuityGrg. 3). Notably, the volume discontinuity is largest in the specimenthat containsthe most Mg; incorporationof Fe in the structureresults in a significant decreasein the magnitude of the discontinuity in the volume at temperature, and Vo is the unit-cell volume extra- the transition. polated to the sametemperature from the data for the high-temperaturephase. The variation of V5 for Volumestrain due to the phasetransition samplesEB and S4-A is shownin Figure 4. Vs is related to the driving order-parameter,Q, for The volume strain due to the ohase transition is a trairsitionby Vs n Q2in lowest order (e.g.,Carpenter defined as 1992).T\e experimentaldata thus provide a prelimi- nary meansof characterizingthe equilibrium evolution vr=v:=vo (t) of Q throughthe cubic to orthorhombictransition. For "vo a symmetry changeF43c <+ Pca2y tncluding a unit- cell doubling, odd terms are permitted in the Landau where V is the measuredunit-cell volume at a siven expansionfor excessfree energy (Dvof6k & Petzelt TRANSITIONS IN BORACITE-GROUP PHASES 889
1971,Stokes & Hatch 1988).The generalform ofthis close to being tricritical (ff * T, V n 7) in this potential (without considering t]le multicomponent interpretation. natureof O) will be that of the standard234 potential: The data clearly are insufficiently preciseto allow any conclusive distinction between the two general G = l/za(T- TdQ2 + 1/tuQ3+ Vqbt Q) forms of governing potential at this stage. It is interesting to note, however, that the trend toward for which the solution for Q @fter Bruce & Cowley ricritical or second-ordercharacter with increasingFe 1981)is: contentcoincides with a decreasein theabsolute values of the volume strains in the three samplesfor which Q = 3/cQo{r+ lr - a/s 0.20 DrscussroN 0.19 ..r/.4..*-, The borate deposits at Sussex, New Brunswick, 0.18 contain the three boracite-group minerals boracite, 0.17 trembathite and congolite. The range of chemical minerals 0.16 compositionsdisplayed by the boracite-group (both bulk crystal and zoning within the samecrystal) 0.15 suggeststhat a complete solid-solution series from 0.14 Mg3B7O13Clto FerBrOtrClmay occur.This studyhas also revealedthe phaserelations in this series. 0.'t3 oloo Any boracite-groupmineral that crystallizesin the 0.12 stability field of the cubic phase will undergo a 0.18 stuctural fransition,or transitionsoupon cooling.These structuraltransitions will inducemicrostructures in the 0.17 aa crystals, explaining why most natural and synthetic crystals of boracite-groupminerals contain multiple o 0.16 complexlamellar (Schmid N t domainsand configurations e- a rl & Tippmann 1978) and display anomalous optical 0.'t5 T t '1, properties. However, a crystal that grew at a 3 temperature below the cubic-to-orthorhombic L 0.14 -.3.;..J' transition in the caseof Mg-rich compositions,or the 0.13 orthorhombic-to-rhombohedraltransition if the crystal "a is Fe-rich, will not have undergonea phasetransition 0.12 upon cooling. Such a crystal will not contain 0.16 transformation-inducedmicrostrucfures. Thus, the thermal history of the borate deposit is recordedby the presence or absenceof transformation-induced 0.15 microstructuresin the boracite-group minerals. The 'lo presenceof transformation-inducedmicrostructures in 0.14 these crystals provides a minimum temperatureof o. l crystallization,whereas an absenceof transformation- 0.13 '\|, inducedmicrostrucfures places an upper bound on the temperatureof crystallization. (1980) 0.12 ri. Herde et al. discussedthe occurrenceof a lr' rhombic form of boracite, which they referred to as oa oostassfurtite'nin the Zechstein deposits of Germany. 0.11 This phasehas never beenaccepted as a valid mineral 0 50 100150 200 250 300 350 400 450 500 species.Examination of Figure 2 indicates that the Temperature(t) rhombohedralstructure will only be stable at 25"C if the structure contains at least 36 mol.Vo FIG.5. Peaktull-width at half-maximum(FWHM) for rhe oostassfurtite" peak conesponding to tlte overlapping (400)ordrorho.bi",FqB7O13Cl,whereas reportedly is pure (040)o,.5ery,e661gand (224)o.no"1or61"reflections in the Mg3BrOr3Cl.Heide et al. (1980)provided DTA curves diffractogramsof sample EB [(Mgr.rrFeo.t7)B7or3cl, for four specimens,including one of "stassfufiite".The bottoml, S4-A [(Mg2.2aFes.7eMne.e5)B7o13Ct,middle] DTA curve for this specimen reveals only one andS8-A [@e1.55Mg1.a5)BrO,.Cltop]. absorption event which occurs at -265"C, exactly where the cubic-to-orthorhombic phase transition occurs in Mg3B7O13Cl.Thus, in light of this study, it seemsvery unlikely that "stassfurtite'is anythingother than boracite.Heide et al. (1980) also reporteda DTA curve for an Fe-rich specimen.The resultsindicate that more than one phasetransition occurson heating.The first transition (or possibly transitions) occurs at becausethe unit cells of the low-symmetryphases do -215"C, whereasthe other @curs at -320oC, values not remain exactly dimensionally cubic. Thus, in good agxeementwith the phase diagram reported althoughthe symmetry-breakingstrain dueto thephase herein Figure 2. transition is small in these phases,it is readily In order for boracite-type phases to be used in observable by examiniag the FWHM of selected applicationsthat take advantageof their ferroelectric diffraction-peaks. and ferroelastic properties, a composition having a TRANSIUONS IN BORACITE-GROUPPHASES 891 (1984); in very small shearangle is requiredto preventcracking. DEVARATAN,V. & SAUE,E, Phasetransitions investigationof the nonlinear dependence AlthoughSchmid & Tippmann(1978) have suggested KrCd2(SO/3: of spontaneoussffain and morphic birefringence on certainboracite-type that solid-solutionseries between order parameteras determinedfrom excess entropy phase end-membersmay exhibit small amounts of measurements.J. Phys. C 17,5525-5537. structural strain, to the best of our knowledge,this is the first study of the phasetransitions in any boracite- Dowry, E. & Cum, J.R. (1973): Crystal-structure type phase of an intermediate composition. In the refinementsfor orthorhombicboracite, Mg3ClBrOl3' and cunentstudy of the seriesMg3BrOl3Cl - FqBTOrrCl, a trigonal, iron-rich analogue.Z Kristallogr. 138' 64-99. both the character of the phase transitions and the structural strains due to the phase transitions are DvoiAr, V, & Pe'rznr,r,J. (1971): Symmetry aspectof significantly dependentupon the composition of the the phase transitions in boracites. Czech.J. Phys. B2L, specimen.Our resultsindicate that the structuralstrain 1t4r-1152. associatedwith the cubic-to-orthorhombictransition F.C. (1994): decreasessignificantly with increasingFe content in GRrcE,J.D., BuRNs,P.C. & HawrnonNs, megastructuresof the borate - FqBrOrrCl. Thus,it may be Determination of the the seriesMg3BTOrrCl polymorphs pringleite and ruitenbergite. Can. Mineral. the possible to attain a desired behavior by altering 32.1-r4. compositionof the boracitephase by substitutionat the metalsite(s). HsroB, K., FneNre, H. & BRUcKNnn,H.-P. (1980): Vorkommen und Eigenschaftenvon Boracit in den AcrolowlepcgMEIlrs Zechsteinsalzlagersdttender DDR. Chem. Erde 39' 201-232. This work was supportedby the Natural Sciences and EngineeringResearch Council of Canadain the &, VoLKscH, G. (L979): Rasterelektro- form of a Post-DoctoralFellowship to PCB. Clare Hall, nenmikroskopischeUntersuchungen zur Morphologievon aus den Cambridge, supported PCB with a Research Boracit, Ascharit und Sulfoborit der DDR. Chenr.Erde 38,223-232. Fellowship. We thank Drs. Gordon J. Nord, Jr. and Zechsteinlagerstlitten their thoughtful reviews of the Peter J. Heaney for HoNEe' R.M. & Bscr, F'R' (1962)l chambersite,a new manuscriptand Robert F. Martin for editorial work that rnneral,Am. Mineral.47"665-67t' improvedthe clarity of this contribution. Iro, T., MoRnvloro,N. & SaoaNaca,R. (1951):The crystal RErenBl.rcEs structureof boracite.Acta Crystallogr. 4, 310-316. Appr.gMAN,D.E. & EvaNs,H.T., Jn. (1973):Iob 9214: KnN, R. & Scnacrs, I. (1955):Vorkommen und analyse indexing and least-squaresrefinement of powder der Boracit- und Ericaitkristalle aus dem Salzhorstvon diffractiondata. U.S. Geol. Sun., Comput.Contrib. X Wathlingen-Hiinigsen. KaIi und SteinsalzlL, 33-42. (NTISDoc. PB2-16188). MANDARTNo,J.A., Racl-rN, A.L., Dnw, P.J.,LE PAcE Y.' Bnucs.A.D. & Cowr.Ev,R.A. (1981):Structural Phase BAcK,M.E., Munowctlcr, 8.L., RAIvni R.A. & FALS, Transitions.Taylor and Francis, London, U.K. R.B. (1990): Redefinition of volkovskite and its descriptionfrom Sussex,New Brunswick. Can. Mineral- Bunrs, P.C. (1995):X-ray powderdiffraction data for the ?a,35r-3s6. identificationof boracite-groupmin erals. Powder Diff. 10, 250-260. MsNnozA-Alvensz, M.-E., YvoN, K., DEPMEER,W. & Scrnan, H. (1985): Structurerefinement of trigonal & CanrnNrsR,M.A. (1996):Phase transitions in iron-chlorine borucite.Acta Crystallogr. C4l, 1551-1552. the seriesboracite - trembathite- congolite:an infrared spectroscopicsntdy. Can. Mineral.34 (in press). NEuvEs,R.J. (1974): Structural studies ofboracites. A review ofthe propertiesofboracites. J. Phys.C: Solid StatePlrys. Gp.rcs.LD. & HewrsonNe,F.C. (1995):Borate 7.3840-3854. minerals.I. Polyhedralclusters and fundamentalbuilding blocks.Caz. Mineral. 33, I 131-1151. RACHLTN.A.L., MANDARINo,J.A., Munowcncr, 8.L., P.J. & BAcK, M.E. (1986): HAWTHoRNE,F.C. & Sunrwc, I.A.R. (1992): Rarr,rrr,R.A., DuNN, hilgardite4M from evaporites in New Trembathite, (Mg,Fe)387O13C1,a new borate mineral Mineralogy of. Mineral. A, 689-693. from the Salt Springs potash deposit, Sussex,New Brunswick. Can. Brunswick. Can. Mineral. 30, 445448. RoBERrs,A.C., Srnr,mc, J.A.R.,GucB, J.D., BunNs'P.C., (1993): CARpENTER,M.A. (1992): Thermodynamics of phase RoursroN, B.V., Cunus, J.D. & Jewon, J.L' polymorphs of ransitions in minerals: a macroscopicapproach. 1rl The Pringleite and ruitenbergite, species Stability of Minerals (G.D. Price & N.L. Ross, eds..r. CaeB26O3a(OH)ztCl4'13H2O,two new mineral Chapman& Hall, London,U.K. (172-215). from Sussex.New Brunswick.Can. Mineral 31,795-800. 892 RoulsroN,B.V. & WAUGH,D.C.E. (1981): A boratemineral & Tnruaux, H. (1978): Spontaneousbirefringence assemblagefrom the Penobsquisand Salt Springs in boracites - measurements and applications. evaporite deposits of southem New Brunswick. Can. Ferroelectrics20, 2l-36. Mineral. 19,291-301,. SroKEs,D.M. & HercH, H.T. (1988): Isotropy Subgroupsof - & - (1983):Srrafigraphic comparison ofthe the 230 CrystallographicSpace Groups. World Scientific, Mississippianpotash deposits in New Brunswick,Canada. 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