Carlsbad-albite methodcentered bisectrix method.ReferencesDec. 3, 2003Mineralogy 231Lecture 36: crustal minerals: plagioclaseThursday: sensors of briny fluids: feldspathoids, scapolitereadings: N 208-228; K 476-484, 549-554; DHZ 473-514reminderfirst draft of term project due Fridayplagioclase groupsolid solution between albite, NaAlSi3O8 (Ab) and anorthite CaAl2Si2O8 (An)specify plagioclase in mole %, e.g.: An47Ab50Or3; e.g., binary plagioclase as An38Ab62 or just An38plagioclase name defined by %An: albite, An0-10oligoclase, An10-30andesine, An30-50laboradorite, An50-70bytownite, An70-90 anorthite, An90-100continuous solid solution at high T for Ab-An but not in ternary Ab-An-Orlow (?) T solvi in plagioclase seriesnames of gaps locus commentperisterite albite-oligoclase cresting at ca. 525°C albite-oligoclase in greenschistsHüttenlocher laboradorite-bytownite at ca. 500-600°C? cause of labradorescenceBøggild bytownite-anorthite at ca. 500-600°C? required by space group changeplagioclase opticsbest described as a function of solid solution and order-disorder rather than by individual speciesRI vs. An content, 2V and sign vs. An content Fig. 12.13albite, andesine-laboradorite have 2V, oligoclase-andesine, bytownite-anorthite –2Vplagioclase twinningalbite, Carlsbad, pericline twin laws common (Ala, Manebach, Baveno twin laws fortunately rare)twinning spacing generally increases with temperature of formationplagioclase is triclinic mineral, so vibration directions X, Y, Z not // to a, b, c Fig. on p. 215extinction positions may vary widely, can provide estimate of composition, sometimes structure infoMichel Lévy method (for albite twins) Fig. 12.15select section vertical to (010) cleavage, measure 6-12 grains, take maximum angle Fig. 12.16Carlsbad-albite methodneed both Carlsbad and albite twinning, four extinction positions observed Fig. 12.17select section vertical to (010) cleavage, measure two extinction angles, cf. graphs Fig. 12.18centered bisectrix method. obtain centered bisectrix figure, Bxa or Bxo, often difficult to distinguish why?measure extinction of cleavage Heinrich, Fig. 12.37useful for untwinned plagioclase, common in medium grade metamorphic rocksuse of determinative graphscurves should be given for both ordered and disordered plagioclaseif rocks are metamorphic or slowly cooled igneous, use ordered curveif rocks are volcanic or “hypabyssal”, use disordered curvefeldspars in contact metamorphic rocks or shallow intrusions may have intermediate ordering stateplagioclase solid solutionsusually close to An-Ab binary (low T, or high T in absence of Kfs) or An-Ab-Or ternary (high T)plagioclase commonly has minor Fe for Al, rarely minor Sr or Ba for Cabarian feldsparshyalophane, Cn15-75Or75-15Ab0-20 (Hy), uncommon in metasediments, granites, rhyolitessolvi between K-rich Hy and Or, Ba-rich Hy and Cn, Hy and Abcelsian, BaAl2Si2O8 (Cn), rare in metasedimentsrelated framework silicate structuresstructure: different space group than feldspars, otherwise similar structuresparacelsian, BaAl2Si2O8 , polymorph of celsian, very rareslawsonite, SrAl2Si2O8 (Sl), very rare, in metasediments, skarnsstructure of slawsonite is not exactly the same as feldspardanburite, CaB2Si2O8, very rare, in metasediments, skarnshurlbutite, CaBe2P2O8, very rare, in pegmatites, iron skarnschemistry is noteworthy: BaSr-1, BAl-1, AlSiBe-1P-1wide solvi between Sl, Cn and An; solvi may close at high Tplagioclase phase equilibria(1) melting T-X diagram for An-Abcontinuous solid solution at high T, An w/ high melting point (mp), Ab w/ low mp DHZ, Fig. 155diagram for An-Ab at low to medium T not well known(2) subsolidus T-X diagram for An-Ab-Orternary system has continuous Ab-An and Ab-Or at high T K, Fig. 11.53significant ternary solutions of plagioclase, alkali feldspars at igneous T (>700°C)can be used as a geothermometer, small correction for P DHZ, Fig. 126diagram perturbed strongly by low temperature solvi, not well known as f(T) K, Fig. 11.56ReferencesDeer, R.A., Howie, W.A. & Zussman, J. (1991) An Introduction to The Rock-Forming Minerals, 2nd ed.,Longman, NY, 696 p.Deer, W.A., Howie, R.A. & Zussman, J. (2001) Vol. 4A, Rock-Forming Minerals: Feldspars, 2nd. ed., TheGeological Society, London, 972 p.Heinrich, E.W. (1965) Microscopic Identification of Minerals, McGraw-Hill, NY, 414 p.Ribbe, P.H., ed. (1983) Feldspar Mineralogy, 2nd ed., Rev. Mineral. 2, 362