TY - JOUR
T1 - Enthalpies of ordering in the plagioclase feldspar solid solution
AU - Carpenter, M. A.
AU - McConnell, J. D.C.
AU - Navrotsky, A.
N1 - Funding Information:
rium and stability. We have deliberatelyt ried to assesst he implications of the enthalpy and experimental resultsi ndependentlyo f previousa ccountso f the plagioclases ubsolidusp haser elations. Interpretation of the many and complex microstructures observedb y transmissione lectron microscopy,f or example,i nvolves a differents et of subjectiveju dge-ments.P erhapst he questionsr aisedh ere,p articularly relating to the sequenceso f ordering, will lead to some reconsiderationo f theseo bservationsb, ut in the future it will obviously be necessaryto reconcile all strandso f the microstructural.p etrographice, x-perimentala nd calorimetrice vidence. Our principal conclusionsa re as follows: Although a greatd eal of data has been generated Francisa nd R. C. Newtonf or generouslpyr ovidingu s with Acknowledgements-We are most gratefutlo P. Gay, C. in this study, the ability to calculatee xactthermo-importants amplesf or this study,a nd to J. V. Smith and dynamic propertiesi s, as yet, rather limited. The G. L. Hovis for theirc riticalc ommentso n the manuscript. problemsa re firstly to separateo ut mixing and or-We alsot hankA . R. Abrahamf or assistancwei th collecting deringc ontributions,s econdlyt o distinguishb etween the X-ray data. Financial supportf rom the Natural Envi- the thermodynamicb ehaviouro f thed ifferentp ossible 4404t o JDCMcC) and fromt heN ationalS cienceF oundation ronmenRt esearcCho uncilof GreatB ritain( grantn o. GR3/ ordereds tructuresa nd, finally, to producep hysically (NSF grantn o. DMR 8 106027to AN) is gratefullya cknowl- realistic,c ontinuous orderingm odelsf or everyc om-edgedT. his is CambridgeE arth SciencesC ontributionn o. position in the solid solution. The data do, however, Es 517. providet he first systematics etr elatingt o the ordering and providea basisf or testingh ypotheseos f equilib-Editorial handling: P. C. Hess
PY - 1985/4
Y1 - 1985/4
N2 - Enthalpies of solution in lead borate at ~700°C have been measured for 36 natural and heat treated plagioclase feldspars. The samples made up two series, as characterised by TEM and XRD. A "low" series contained the natural ordered material and a "high" series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions: low albite → high albite: ~3 kcal/mole "e" structure → Cī high albite structure: ~ 1.4-2.8 kcal/mole Il̄ structure →- Cl̄ high albite structure: ~0.7-1.9 kcal/mole Il̄ structure equilibrated at low temperature → Il̄ structure equilibrated at high temperature: ~ 1.8-0.8 kcal/mole. ΔHsoln data for the high series overlap with the data of Newton et al. (1980) for synthetic high structural state plagioclases except in the composition range ~An90-An100. They are consistent with an interpretation of the solid solution as being composed, at high temperatures, of two ideal (zero heat of mixing) segments, one with Cl̄ symmetry and one with Il symmetry, and having a non-first order (continuous) order/disorder transformation between them. The low series can also be separated into two distinct trends, for Il̄ and "e" structures. Values of the enthalpy change due to disordering (ΔHord) also show a number of systematic trends. Firstly, the values for e → Cl̄ are larger than for Il̄ → Cl̄ in the composition range where both e and Il̄ structures are observed (~An65-An72). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in ΔHord, which occurs at ~An50, may be important for the origin of the Bøggild miscibility gap. Thirdly, the large enthalpy change of the e structure, due to ordering, may be sufficient to stabilise it relative even to a mixture of low albite plus anorthite. Values for the enthalpy change on disordering Il̄ anorthites and bytownites to a Cl̄ structure have been estimated by assuming that the Cl̄ solid solution is ideal (non-enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure anorthite.
AB - Enthalpies of solution in lead borate at ~700°C have been measured for 36 natural and heat treated plagioclase feldspars. The samples made up two series, as characterised by TEM and XRD. A "low" series contained the natural ordered material and a "high" series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions: low albite → high albite: ~3 kcal/mole "e" structure → Cī high albite structure: ~ 1.4-2.8 kcal/mole Il̄ structure →- Cl̄ high albite structure: ~0.7-1.9 kcal/mole Il̄ structure equilibrated at low temperature → Il̄ structure equilibrated at high temperature: ~ 1.8-0.8 kcal/mole. ΔHsoln data for the high series overlap with the data of Newton et al. (1980) for synthetic high structural state plagioclases except in the composition range ~An90-An100. They are consistent with an interpretation of the solid solution as being composed, at high temperatures, of two ideal (zero heat of mixing) segments, one with Cl̄ symmetry and one with Il symmetry, and having a non-first order (continuous) order/disorder transformation between them. The low series can also be separated into two distinct trends, for Il̄ and "e" structures. Values of the enthalpy change due to disordering (ΔHord) also show a number of systematic trends. Firstly, the values for e → Cl̄ are larger than for Il̄ → Cl̄ in the composition range where both e and Il̄ structures are observed (~An65-An72). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in ΔHord, which occurs at ~An50, may be important for the origin of the Bøggild miscibility gap. Thirdly, the large enthalpy change of the e structure, due to ordering, may be sufficient to stabilise it relative even to a mixture of low albite plus anorthite. Values for the enthalpy change on disordering Il̄ anorthites and bytownites to a Cl̄ structure have been estimated by assuming that the Cl̄ solid solution is ideal (non-enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure anorthite.
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U2 - 10.1016/0016-7037(85)90310-2
DO - 10.1016/0016-7037(85)90310-2
M3 - Article
AN - SCOPUS:0022007735
SN - 0016-7037
VL - 49
SP - 947
EP - 966
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 4
ER -