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Abstract
The concept of myrmekite development by exsolution processes akin to and connected with exsolution perthite formation is finding increasing support. An attempt is made to develop, in terms of solid-state transformational stages, a theoretical model for the process which explains the known features of the exsolution myrmekites and the related perthites. A high-temperature alkalic ternary feldspar is considered to be a heterotype solid-solution, the calcium, solute being vacancy stabilised. The importance of the solute-vacancy association to diffusion is discussed. Initial exsolution at lattice disarray sites is activated by the decreasing stable concentration of vacancies with decreasing temperature. Available lattice disarray sites are internal dislocation type defects and the crystal periphery and at these sites there is nucleation and growth of plagioclase and free quartz as vacancies are annihilated:
Ca++
>(AISi3O8)2 · CaAl2Si2O8 + 4SiO2. vac.
The product is vein perthite with associated quartz blebs at internal sites and, given suitable interfacial componental conditions, myrmekite at the periphery.
Subsequent perthite exsolution is considered to take place by a multi-stage process approximating to precipitation hardening in metal alloys. There is no quartz release by this process.
The nature of the prerequisite interfacial conditions for myrmekite growth are described and suggested to be (a) sufficiency of available interfacial energy, and (b) a suitable foundation for growth of the new plagioclase phase (ideally another plagioclase but acceptably another alkali feldspar)