Contributions to Mineralogy and Petrology

, Volume 115, Issue 1, pp 58–65 | Cite as

An exsolution silica-pump model for the origin of myrmekite

  • Robert O. Castle
  • Donald H. Lindsley


Myrmekite, as defined here, is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase (calcic albite-oligoclase), intimately associated with potassium feldspar in plutonic rocks of granitic composition. Hypotheses previously invoked in explanation of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement. However, several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases. Exsolution of potassium feldspar system projected from [AlSi2O8] involves the exchange CaAlK-1Si-1, in which the AlSi-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary feldspar system is open to excess silica, the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net-transfer reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is concomitantly incorporated in this same phase. Thus silica is “pumped” into the reaction volume from a “silica reservoir”, a process that enhances redistribution of both Si and Al through the exsolving ternary feldspar.


Crystallization Quartz Exchange Reaction Mineral Resource Reaction Volume 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Barker DS (1970) Compositions of granophyre, myrmekite, and graphic granite. Bull Geol Soc Am 81:3339–3350Google Scholar
  2. Becke F (1908) Über Myrmekit. Mineral Petrogr Mitt 27:377–390Google Scholar
  3. Castle RO (1966) Origin of myrmekite (abstract 1965). Geol Soc Am Spec Pap 87:198Google Scholar
  4. Dymek RF, Schiffries CM (1987) Calcic myrmekite: possible evidence for the involvement of water during the evolution of andesine anorthosite from St-Urbain, Quebec. Can Mineral 25:291–319Google Scholar
  5. Fuhrman ML, Lindsley DH (1988) Ternary-feldspar modeling and thermometry. Am Mineral 73:201–215Google Scholar
  6. Fuhrman ML, Frost BR, Lindsley DH (1988) Crystallization conditions of the Sybille Monzosyenite, Laramie Anorthosite complex, Wyoming. J Petrol 29:699–729Google Scholar
  7. Hopson RF, Ramseyer K (1990) Cathodoluminescence microscopy of myrmekite. Geology 18:336–339Google Scholar
  8. Johannsen A (1939) A descriptive petrography of the igneous rocks. I. Introduction, textures, classifications and glossary, 2nd edn. University of Chicago Press, ChicagoGoogle Scholar
  9. Longhi J, Hays JF (1979) Phase equilibria and solid solution along the join CaAl2Si2O8−SiO2. Am J Sci 279:876–890Google Scholar
  10. Nekvasil H (1991) Ascent of felsic magmas and formation of rapakivi. Am Mineral 76:1279–1290Google Scholar
  11. Orville PM (1972) Plagioclase cation exchange equilibria with aqueous chloride solution: results at 700° C and 2000 bars in the presence of quartz. Am J Sci 272:234–272Google Scholar
  12. Phillips ER (1974) Myrmekite — one hundred years later. Lithos 7:181–194Google Scholar
  13. Schiffries CM, Dymek RF (1985) Calcic myrmekite in anorthositic and gabbroic rocks (abstract). Geol Soc Am Abstr Program 1985:709Google Scholar
  14. Schwantke A (1909) Die Beimischung von Ca im Kalifeldspat und die Myrmekitbildung. Centralbl Mineral: 311–316Google Scholar
  15. Sederholm JJ (1916) On synantetic minerals and related phenomena. Bull Commun Geol Finland 48Google Scholar
  16. Spencer E (1945) Myrmekite in graphic granite and in vein perthite. Mineral Mag 27:79–98Google Scholar
  17. Thompson JB (1982a) Composition space: an algebraic and geometric approach. In: Ferry JM (ed) Characterization of metamorphism through mineral equilibria. (Reviews in mineralogy 10) Mineral Soc Am, Washinton DC, pp 1–31Google Scholar
  18. Thompson JB (1982b) Reaction space: an algebraic and geometric approach. In: Ferry JM (ed) Characterization of metamorphism through mineral equilibria. (Reviews in mineralogy 10) Mineral Soc Am, Washington DC, pp 33–51Google Scholar
  19. Tronquoy MR (1912) Origine de la myrmekite. Bull Soc Mineral Fr 35:214–223Google Scholar
  20. Turi B, Taylor HP Jr (1971) An oxygen and hydrogen isotope study of a granodiorite pluton from the Southern California Batholith. Geochim Cosmochim Acta 35:383–406Google Scholar
  21. Tuttle OF (1952) Origin of the contrasting mineralogy of extrusive and plutonic salic rocks. J Geol 60:107–124Google Scholar
  22. Tuttle OF, Bowen NL (1958) Origin of granite in the light of experimental studies in the system NaAlSi3O8−KAlSi3O8−SiO2 −H2O. Geol Soc Am Mem 74Google Scholar
  23. Widenfalk L (1969) Electron micro-probe analyses of myrmekite plagioclases and coexisting feldspars. Lithos 2:295–309Google Scholar
  24. Yund RA (1983) Diffusion in feldspars. In: Ribbe PH (ed) Feldspar mineralogy, 2nd edn. (Reviews in mineralogy 2) Mineral Soc Am, Washington DC, pp 203–222Google Scholar

Copyright information

© Springer-Verlag 1993

Authors and Affiliations

  • Robert O. Castle
    • 1
  • Donald H. Lindsley
    • 2
  1. 1.U.S. Geological SurveyMenlo ParkUSA
  2. 2.Dept. Earth and Space SciencesState University of New York at Stony BrookStony BrookUSA

Personalised recommendations