Lecture 38: sensors of briny fluids: feldspathoids, scapoliteReferencesDec. 4, 2003Mineralogy 231Lecture 38: sensors of briny fluids: feldspathoids, scapoliteFriday: molecular sieves: water softeners, water carriers: zeolitesreadings: N 227-233; K 486-489, 558-562; DHZ 515-529feldspathoid groupframework silicates, chemically similar to feldspars but with lower silica contentslarger structural cavities on alkali sites, lower density, negative relief relative to feldsparsno octahedral sites, therefore little Fe, no Mg, as is also the case for feldsparsfluorite, zeolites only other common minerals with strong negative reliefoccurrences of feldspathoidsalkali-rich (Na-, K-rich) igneous rocks, low in silica (feldspathoids not stable with SiO2) intrusive rocks: nepheline syenites, some nepheline-bearing carbonatitesMt. Vesuvius, Mt. Etna, late-stage lava flows on Oahu, other ocean island basalts (OIBs)some feldspathoids (and scapolite) take up chloride, carbonate, sulfateleucite: KAlSi2O6cubic at high T, transforms to tetragonal twinned structure below 600°Cwhat is more stable than leucite + quartz?found mainly in alkaline volcanic rocks as a phenocryst: Leucite Hills WY, East Africa volcanicsvery low RI, complexly twinned Fig. on p. 227structure has KXII in large cavities, AlIV, SiIVK, Fig. 11.58there are two sets of tetrahedral rings, a four-ring and a six-ringnepheline: NaAlSiO4Na3KAl4Si4O16 is better formula (used by Nesse): most natural nepheline has 20-40% Ksolid solution toward kalsilite, KAlSiO4 (kalsilite misspelled by Nesse)up to 20% solid solution of [ ]0.5Ca0.5AlSiO4 (chemically equivalent to anorthite (CaAl2Si2O8)chemical exchange is [ ]0.5Ca0.5Na-1what is more stable than nepheline + quartz?nepheline common in alkalic volcanic rocks and syenites, sometimes found in gneisseshexagonal, uniaxial negative, low RI, very low birefringence () Fig. on p. 227structure similar to tridymite (SiO2 polymorph), has (Na,K)XII in large cavities, AlIV, SiIVK, Fig. 11.58sodalite: Na4Al3Si3O12Cl (3Ne.NaCl)related minerals: nosean with SO4, haüyne with S, SO4often blue in hand specimen, colorless or faintly blue in thin section, cubic, v. low RI Fig. on p. 227Cl anions in large cavities surrounded by 4 Na cationssodalite syenites:shallow intrusives in Monteregian Hills, Tahiti volcanic rocksigneous sodalite often white dodecahedral crystalswhat is more stable than sodalite + quartz?nepheline transformed to sodalite in the presence of aqueous solutions with dissolved NaClcancrinite: Na3CaAl3Si3O12CO3 (3Ne.CaCO3)not listed in Nesse or Klein, hexagonal, often yellow in hand specimen and thin sectionlow RI, uniaxial negative, low RI, med , what is more stable than cancrinite + quartz?nepheline transformed to cancrinite in the presence of aqueous solutions with dissolved CaCO3scapolite: Na-Ca-Al silicate with Cl, CO3, SO4not usually classified as a feldspathoid but has chemical similaritiesscapolite is stable with quartzmeionite: Ca4Al6Si6O24CO3 (3An.CaCO3): granulite-facies marbles, meta- anorthosites, metagabbrosscapolite often forms rims on plagioclase by reaction with calcite, hydrothermal fluidsmarialite: Na4Al3Si9O24Cl (3Ab.NaCl), in sodic rocks exposed to briny fluidssilvialite: Ca4Al6Si6O24SO4 (3An.CaSO4), in granulites at high P and Tcontinuous solid solution between meionite, marialite and silvialitemany intermediate Na-Ca scapolites are Cl- and SO4-free (“mizzonite”)tetragonal, uniaxial negative, low to medium RI, low to medium birefringence Fig. on p. 232structure has Cl anions in large cavities surrounded by 4 Na cations K, Fig. 12.84phase equilibria for NaAlSiO4 – SiO2subsolidus phase diagram for nepheline, albite, jadeite, quartz DHZ, Fig. 77phase equilibria for KAlSiO4 – SiO2system contains kalsilite, leucite, K-feldspar, quartz DHZ, Fig. 139phase equilibria for NaAlSiO4 – KAlSiO4asymmetric solvus between nepheline and kalsilite K, Fig. 12.83potassian nepheline exsolves kalsilite upon coolingphase equilibria for CaAl2Si2O8 – CaCO3 and CaAl2Si2O8 – CaSO4anorthite and calcite react to form meionite above 750°C DHZ, Fig. 190aanorthite and anhydrite react to form silvialite at high P and/or T DHZ, Fig. 190bNa and Cl solid solution stabilizes scapolite to lower T (some marialite forms at 400-500°C)phase equilibria for NaAlSi3O8 – CaAl2Si2O8 – CaCO3sketchsystem shows plagioclase solid solution (ss) and simplified NaCl-free scapolite sstwo three-phase regions: calcite-“carbonate marialite”-oligoclase, calcite-meionite-anorthitephase equilibria for NaAlSiO4 – NaClsystem shows nepheline, sodalite, halite Sharp et al. (1989)with decreasing a(NaCl), sodalite transforms to nephelinewith increasing a(NaCl), nepheline transforms to sodalitesodalite often forms veins when briny fluids circulate through nepheline-bearing rocki.e., Princess Sodalite Mine, near Bancroft, OntarioReferencesDeer, R.A., Howie, W.A. & Zussman, J. (1991) An Introduction to The Rock-Forming Minerals, 2nd ed.,Longman, NY, 696 p.Sharp, Z.D., Helffrich, G.R., Bohlen, S.R. & Essene, E.J. (1989) The thermodynamics and phaseequilibria of sodalite. Geochim. Cosmochim. Acta 53,