Springer Nature is making SARS-CoV-2 and COVID-19 research free. View research | View latest news | Sign up for updates

An exsolution silica-pump model for the origin of myrmekite


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.

This is a preview of subscription content, log in to check access.


  1. Barker DS (1970) Compositions of granophyre, myrmekite, and graphic granite. Bull Geol Soc Am 81:3339–3350

  2. Becke F (1908) Über Myrmekit. Mineral Petrogr Mitt 27:377–390

  3. Castle RO (1966) Origin of myrmekite (abstract 1965). Geol Soc Am Spec Pap 87:198

  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 25:291–319

  5. Fuhrman ML, Lindsley DH (1988) Ternary-feldspar modeling and thermometry. Am Mineral 73:201–215

  6. Fuhrman ML, Frost BR, Lindsley DH (1988) Crystallization conditions of the Sybille Monzosyenite, Laramie Anorthosite complex, Wyoming. J Petrol 29:699–729

  7. Hopson RF, Ramseyer K (1990) Cathodoluminescence microscopy of myrmekite. Geology 18:336–339

  8. Johannsen A (1939) A descriptive petrography of the igneous rocks. I. Introduction, textures, classifications and glossary, 2nd edn. University of Chicago Press, Chicago

  9. Longhi J, Hays JF (1979) Phase equilibria and solid solution along the join CaAl2Si2O8−SiO2. Am J Sci 279:876–890

  10. Nekvasil H (1991) Ascent of felsic magmas and formation of rapakivi. Am Mineral 76:1279–1290

  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 272:234–272

  12. Phillips ER (1974) Myrmekite — one hundred years later. Lithos 7:181–194

  13. Schiffries CM, Dymek RF (1985) Calcic myrmekite in anorthositic and gabbroic rocks (abstract). Geol Soc Am Abstr Program 1985:709

  14. Schwantke A (1909) Die Beimischung von Ca im Kalifeldspat und die Myrmekitbildung. Centralbl Mineral: 311–316

  15. Sederholm JJ (1916) On synantetic minerals and related phenomena. Bull Commun Geol Finland 48

  16. Spencer E (1945) Myrmekite in graphic granite and in vein perthite. Mineral Mag 27:79–98

  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 1–31

  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 33–51

  19. Tronquoy MR (1912) Origine de la myrmekite. Bull Soc Mineral Fr 35:214–223

  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 35:383–406

  21. Tuttle OF (1952) Origin of the contrasting mineralogy of extrusive and plutonic salic rocks. J Geol 60:107–124

  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 74

  23. Widenfalk L (1969) Electron micro-probe analyses of myrmekite plagioclases and coexisting feldspars. Lithos 2:295–309

  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 203–222

Download references

Author information

Correspondence to Robert O. Castle.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Castle, R.O., Lindsley, D.H. An exsolution silica-pump model for the origin of myrmekite. Contr. Mineral. and Petrol. 115, 58–65 (1993). https://doi.org/10.1007/BF00712978

Download citation


  • Crystallization
  • Quartz
  • Exchange Reaction
  • Mineral Resource
  • Reaction Volume