On the origin of alkali feldspar megacrysts in granitoids: the case against textural coarsening

  • Guilherme A. R. GualdaEmail author
Original Paper


Alkali feldspar megacrysts (up to 20 cm in length) are common in granitoids, but their origin has been a matter of continued debate. In particular, the relative role of various magmatic and subsolidus processes has proven difficult to assess given the protracted crystallization history of these megacrysts. Textural coarsening (also known as Ostwald ripening) has been invoked to explain the extraordinarily large sizes of these feldspar megacrysts. While coarsening is a thermodynamically spontaneous process, it is important to evaluate the timescales in which it operates during magmatic evolution. In this work, we consider theories of coarsening of a second phase in the melt in binary systems to evaluate if coarsening can explain the origin of alkali feldspar megacrysts in granitoids. The theories explored are all approximate and somewhat incomplete; however, all of them predict the same basic kinetic equation for the coarsening process. Calculation of the fundamental rate constant controlling the coarsening processes requires determination of parameters that are not particularly well constrained for minerals in silicate melts. Nonetheless, we demonstrate that diffusion rates are the main control on the rate constant for coarsening. Using a range of rate constants calculated using suitable parameters for silicate magmatic systems, we show that coarsening in silicate magmas cannot lead to crystals larger than tens of µm on timescales relevant for igneous processes. The timescales required to generate millimeter to centimeter crystals by coarsening are so prohibitively large that the weaknesses of the existing kinetic theories becomes unimportant, and the idea of generating megacrysts via textural coarsening can be conclusively abandoned. The origin of feldspar megacrysts probably results from the interplay between nucleation and growth rates in magmas, a problem that has not been dealt in detail in the literature.


Megacrysts Textural coarsening Ostwald ripening Granitoids 



This project started as a group project involving students attending a graduate course taught by Gualda in 2011. Several students participated in the initial stages of the project, and their input is greatly appreciated: Abraham Padilla, Ayla Pamukcu, Steven Braun, and Timothy Peters. Comments on earlier versions of the manuscript by Susanne Seitz and Lydia Harmon helped clarify several portions of the manuscript. Comments and suggestions by two anonymous reviewers and editorial handling by Mark Ghiorso are greatly appreciated.


  1. Ardell AJ (1972) Effect of volume fraction on particle coarsening: theoretical considerations. Acta Metall 20(1):61. CrossRefGoogle Scholar
  2. Ardell AJ (1987) Precipitate coarsening in solids: modern theories, chronic disagreement with experiment. In: Lorimer GW (ed) Phase Transformations. Institute of Metals, London, pp 485–494Google Scholar
  3. Ardell AJ, Nicholson RB (1966) Coarsening of gamma in Ni–Al alloys. J Phys Chem Solids 27(11–1):1793. CrossRefGoogle Scholar
  4. Baker DR (1991) Interdiffusion of hydrous dacitic and rhyolitic melts and the efficacy of rhyolite contamination of dacitic enclaves. Contrib Miner Petrol 106(4):462–473CrossRefGoogle Scholar
  5. Barboni M, Schoene B (2014) Short eruption window revealed by absolute crystal growth rates in a granitic magma. Nat Geosci 7(7):524–528. CrossRefGoogle Scholar
  6. Carlson WD (1999) The case against Ostwald ripening of porphyroblasts. Can Mineral 37:403–413Google Scholar
  7. Christian JW (2002) The theory of transformations in metals and alloys: an advanced textbook in physical metallurgy. Pergamon Press, Oxford, New YorkGoogle Scholar
  8. Collins WJ, Wiebe RA, Healy B, Richards SW (2006) Replenishment, crystal accumulation and floor aggradation in the megacrystic Kameruka Suite, Australia. J Petrol 47(11):2073–2104. CrossRefGoogle Scholar
  9. Dehoff RT (1991) A geometrically general theory of diffusion controlled coarsening. Acta Metall Mater 39(10):2349–2360. CrossRefGoogle Scholar
  10. Dowty E (1980) Crystal growth and nucleation theory and the numerical simulation of igneous crystallization. In: Hargraves RB (ed) Physics of magmatic processes. Princeton University Press, PrincetonGoogle Scholar
  11. Gagnevin D, Daly JS, Poli G, Morgan D (2005) Microchemical and Sr isotopic investigation of zoned K-feldspar megacrysts: insights into the petrogenesis of a granitic system and disequilibrium crystal growth. J Petrol 46(8):1689–1724. CrossRefGoogle Scholar
  12. Ghiorso MS, Carmichael ISE, Moret LK (1979) Inverted high-temperature quartz: unit-cell parameters and properties of the alpha-beta inversion. Contrib Miner Petrol 68(3):307–323. CrossRefGoogle Scholar
  13. Glazner A, Johnson B (2013) Late crystallization of K-feldspar and the paradox of megacrystic granites. Contrib Miner Petrol 166(3):777–799. CrossRefGoogle Scholar
  14. Gualda GAR, Pamukcu AS, Ghiorso MS, Anderson AT Jr, Sutton SR, Rivers ML (2012) Timescales of quartz crystallization and the longevity of the Bishop giant magma body. PLoS One 7(5):e37492. CrossRefGoogle Scholar
  15. Hardy SC, Voorhees PW (1988) Ostwald ripening in a system with a high volume fraction of coarsening phase. Metall Trans Phys Metall Mater Sci 19(11):2713–2721. CrossRefGoogle Scholar
  16. Higgins MD (1999) Origin of megacrysts in granitoids by textural coarsening: a crystal size distribution (CSD) study of microcline in the Cathedral Peak Granodiorite, Sierra Nevada, California. Geol Soc Lond Spec Publ 168(1):207–219. CrossRefGoogle Scholar
  17. Johnson BR, Glazner AF (2010) Formation of K-feldspar megacrysts in granodioritic plutons by thermal cycling and late-stage textural coarsening. Contrib Miner Petrol 159(5):599–619. CrossRefGoogle Scholar
  18. Jurewicz SR, Watson EB (1984) Distribution of partial melt in a felsic system: the importance of surface-energy. Contrib Mineral Petrol 85(1):25–29. CrossRefGoogle Scholar
  19. Kuehmann CJ, Voorhees PW (1996) Ostwald ripening in ternary alloys. Metall Mater Trans Phys Metall Mater Sci 27(4):937–943. CrossRefGoogle Scholar
  20. Lifshitz IM, Slyozov VV (1961) The kinetics of precipitation from supersaturated solid solutions. J Phys Chem Solids 19(1–2):35–50. CrossRefGoogle Scholar
  21. Moore JG, Sisson TW (2008) Igneous phenocrystic origin of K-feldspar megacrysts in granitic rocks from the Sierra Nevada batholith. Geosphere 4(2):387–400. CrossRefGoogle Scholar
  22. Pitcher WS (1997) The nature and origin of granite. Chapman & Hall, London, New YorkCrossRefGoogle Scholar
  23. Shewmon PG (1969) Transformations in metals. McGraw-Hill, New YorkGoogle Scholar
  24. Vernon RH (1986) K-feldspar megacrysts in granites—phenocrysts, not porphyroblasts. Earth Sci Rev 23(1):1–63. CrossRefGoogle Scholar
  25. Vernon RH, Paterson SR (2006) Mesoscopic structures resulting from crystal accumulation and melt movement in granites. Trans R Soc Edinb Earth Sci 97:369–381CrossRefGoogle Scholar
  26. Vernon RH, Paterson SR (2008) How late are K-feldspar megacrysts in granites? Lithos 104(1–4):327–336. CrossRefGoogle Scholar
  27. Voorhees PW (1992) Ostwald ripening of two-phase mixtures. Annu Rev Mater Sci 22:197–215. CrossRefGoogle Scholar
  28. Wagner C (1961) Theorie der Alterung von Niederschlägen durch Umlösen (Ostwald-Reifung). Zeitschrift für Elektrochemie, Berichte der Bunsengesellschaft für physikalische Chemie 65(7–8):581–591Google Scholar

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© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Earth and Environmental SciencesVanderbilt UniversityNashvilleUSA

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