Wednesday: review session old examsReferencesDecember 5, 2003Mineralogy 231Lecture 39: molecular sieves: water softeners: zeolitesMonday: classical study in polymorphism: silica polymorphsWednesday: review session old examsreadings: N 201-208; K 475-476, 543-548; DHZ 457-472zeolitesframework silicates chemically similar to feldspars but with water molecules, exchangeable cationsno octahedral sites, therefore very little Fe, no Mg in structure, as is the case for feldsparscontain zeolite substitution: [ ]Ca = 2(Na,K) as well as feldspar substitutions CaAl = NaSi and Na = Kabout 45 zeolite minerals known, hundreds of synthetic species Table 12.5structures with open channels where Na, K, Ca ions and H2O molecules are found Fig. 12.22variable order-disorder of IV Al and Si is likelylow density means they not stable at high P, most are stable only to 2 or 3 kbarmany probably metastable, forming for kinetic reasons at low PT from rhyolitic glass or in veinslow birefringence, lower density -> low RI (negative relief relative to feldspars)fluorite, feldspathoids only other common minerals with substantial negative relieffound in igneous rocks and sediments subjected to low grade metamorphismalso common in hot springs settingsoften very fine-grained, easily damaged with electron beam, difficult to analyze except with XRDfortunately, most zeolites have very distinctive XRD patternscoarser zeolites found in amygdules (filled vesicles) in volcanic rockspatterns of zeolites with burial depth established for both rhyolitic and basaltic sequencesclassification of zeolitesdistinguished chemically by dominant alkali/alkaline earth element: Na, K, Ca, Ba, Csalso distinguished as silica-rich vs. silica poor, distinguished structurally by powder XRD patterndistinguishing properties of common zeolitesmineral system optics chemistry habit cleavage(s)analcime cub isotropic Na-rich dodecahedra nonechabazite tkl us. – sign Ca-rich (+Na, K) distorted cubes rhombohedral (3)heulandite mkl l ±, incl. extinction Ca-rich (+Na, K) tabular, granular (010)laumontite mkl l +, incl. extinction Ca-rich prismatic, fibers (010), {110}natrolite ort l +, // extinction Na-rich fibers, prismatic {110}stilbite tkl l -, // ext on (100) Ca, Na sheafs, tabular (010)thomsonite ort l ±, // extinction Ca, Na fibrous, prismatic (010)analcime: NaAlSi2O6.H2O (= Jd.H2O, or Ab.Ne.2H2O)not regarded as a true zeolite by some mineralogists; little exchange capabilityhas higher thermal stability (500-550°C in the absence of quartz), higher baric stability (8-10 kbar)may have excess Si via substitution of [ ]Si = NaAl; also shows some [ ]Ca = 2Na, rarely Cs = Na (!)Ca-Al silicates with H2Olaumontite: Ca-Al silicate with H2O: CaAl2Si4O12.4H2O (An.2Qz.4H2O) Fig. on p.295may have Na substitution, stable with quartzcommon in basaltic rocks metamorphosed to low grades (zeolite and prehnite-pumpellyite faciestabular crystals, easily confused with prehnite, lawsonite opticallywairakite, Ca-equivalent to analcime, CaAl2Si4O12.2H2O, obtained by exchange [ ]Ca = 2Nafound in hot springs at Wairaki, NZ, has some analcite solid solutioncubic, very low RI, often forms trapezohedral forms (24 sides) Fig. on p.290pollucite is a rare Cs analcime, found in a Cs-rich pegmatite in Canada, some Cs in analcimeshas some Rb, K solid solutionchabazite: CaAl2Si4O12.6H2O (An.2Qz.6H2O) Fig. on p.293solid solution with variable Na, K, Sr, Ba, stable with quartzheulandite: CaAl2Si7O18.6H2O (An.5Qz.6H2O) Fig. on p.294variable solid solution with Na, K, mainly via [ ]Ca = 2(Na,K); stable with quartznatrolite: Na-Al silicate with H2O: Na2Al2Si3O10.2H2O (Ab.2Ne.2H2O) Fig. on p.291may have some K for Na, very low in Ca, not stable with quartzcommon in basalts, greywackes forming fibrous veinsNa-Ca-Al silicates with H2Ostilbite: NaCa2Al5Si13O36.14H2O (Ab.2An.6Qz.14H2O) Fig. on p.293variable Na/Ca, stable with quartzthomsonite: Na-Ca-Al silicate with H2O: NaCa2Al5Si5O20.6H2O (Ne.2An.6H2O) Fig. on p.292some thomsonite has only Ca; variable Na/Ca, not stable with quartzphase equilibria for CaAl2O4 – SiO2 – H2Ovarious Ca-zeolites related by simple dehydration reactions producing quartz and H2Ochabazite = heulandite at 100°Cheulandite = laumontite + quartz + H2O at ca. 200°Claumontite = wairakite + quartz + H2O at ca. 350°Cwairakite = anorthite + quartz + H2O at ca. 400°Cexact T depends on solid solutions in zeolitessequence of reactions marks increasing metamorphic grade in basalts metamorphosed at low gradesmany other reactions in system involving prehnite, clinozoisite, lawsonitephase equilibria for NaAlO2 – SiO2 – H2Oanalcime + quartz = albite + H2O at about 180°C on the critical curve for H2O DHZ, Fig. 192boundary is taken as the upper limit of zeolite faciesanalcime = nepheline + albite + H2O at about 500-550°Csuggests that nepheline gneisses may react with aqueous fluid to produce late analciteanalcime = jadeite + H2O at about 8-10 kbarReferencesDeer, W.A., Howie, R.A. & Zussman, J. (1962) Vol. 4, Rock-Forming Minerals: Framework Silicates,Longmans, Green & Co., London, 435 p.Deer, R.A., Howie, W.A. & Zussman, J. (1991) An Introduction to The Rock-Forming Minerals, 2nd ed.,Longman, NY, 696