Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks

Abstract

Homogeneous and heterogeneous phase relationships in the alkali feldspars are reviewed, and behaviour diagrams developed. A1,Si ordering is almost certainly continuous and higher order in both albite and potassium feldspar and has been established reversibly or nearly so down to below 500 ~ in albite and possibly to ~ 200 ~ in potassium feldspar. The degree of order in intermediate albite changes strongly over a range of ~ 75-150 ~ depending on pressure, low albite being stable up to about 620-650 ~ and high albite above about 725 ~ at low pressure. Symmetry is broken at ~ 980 ~ mainly by a cooperative shearing of the whole framework and not by A1,Si ordering alone; there is a thermal crossover near 700~ shearing being dominant above (high albite) and ordering dominant below (intermediate albite).In potassium feldspar symmetry is broken by A1,Si ordering at a temperature of about 500 ~ The change in degree of order with respect to temperature has been followed easily and reversibly in sanidine from ~ 1075 to ~ 550 ~ and to a lesser extent in microcline from 450 to 200 ~ Ordering rates in sanidine down to 500 ~ and ordering rates in microcline between 450 and 200 ~ are almost as fast as in albite. Ordering in sanidine at 500 ~ and below slows and then stops with the development of the tweed orthoclase domain texture. The tweed texture acts as a barrier to further order because the strain energy associated with the (incipient) twin domain texture balances or nearly balances the free energy decrease resulting from ordering. Ordering stops not because of the kinetics of A1,Si diffusion, but because the total driving force is very small or nil. Ordering can readily proceed to completion, with the formation of low microcline, only if the domain-texture barrier is overcome by processes involving fluids or strong external stresses. There is no barrier in albite.The symmetry-breaking process in alkali feldspar changes with composition from mainly shearing in albite to ordering in potassium feldspar. Symmetry is broken equally at a compositional crossover (metastable with respect to exsolution) near Abso 75 at low pressure and progressively displaced towards Or at higher pressures. Ordering in pure albite occurs by a (nearly) one-step path which progressively becomes two-step with substitution of Or. Diagrams showing the near-equilibrium variation of the order parameters at low pressure with composition and T are given, as well as two extreme phase and behaviour diagrams for complete coherent and complete incoherent (strain-free) relationships, These diagrams can be used to understand feldspar relationships and microtextures in hypersolvus and subsolvus rocks, the occurrence of orthoclase, and of intermediate and low microcline.

Structural State of K-Feldspar in the Felsic Volcanic Rocks and Ring Pluton Granites of the Butler Hill Caldera, St. Francois Mountains, Southeastern Missouri

“…Because microcline is the thermodynamically stable state for K-feldspar under ambient conditions, one might expect re-equilibration to highly ordered structures to be the norm for the exposed portions of ancient igneous complexes. However, in the absence of deuteric, hydrothermal, or metamorphic conditions, the driving force for the orthoclase-tomicrocline transition is very low (Brown & Parsons 1989), and K-feldspar with intermediate to low structural order has been documented from a number of unaltered Precambrian igneous complexes (Martin & Falster 1986, Schandl et al 1986, Stevenson & Martin 1988. Therefore, we interpret the homogeneity in structural state of the K-feldspar across the entire Butler Hill Caldera complex to be evidence for a pervasive alteration event.…”

“…For such K-rich compositions to result from subsolidus exsolution would require thermal equilibrium to be maintained to rather low temperature (probably 200 to 300°C, depending on the pressure) (Martin 1974, Yund & Tullis 1983. In turn, if thermal equilibrium had been maintained at such low temperature for even a moderately extended period of time, especially in the presence of metasomatic fluids, one would expect the alkali feldspar to have attained very nearly complete structural order (Z = 1.00) (Brown & Parsons 1989). The fact that the alkali feldspar throughout the Butler Hill Caldera shows high but not maximum structural order suggests that the structural state of these feldspars was homogenized by thermal annealing at a moderately high subsolidus temperature, and that the compositions were subsequently purified by lower-temperature hydrothermal/metasomatic alteration.…”

Mecanismos de construcción-destrucción de patrones macroestructurales en microclimas como sistemas complejos

“…una explicación habitual es que el descenso en energía libre del cristal por orden Si/Al es muy inferior a la energía de activación necesaria relacionada con la rotura de enlaces Si-O y Al-O. Además, la formación de dominios con secuenciación orden-antiorden podría ser un carácter adicional que inhibe el engrosamiento de los dominios (18). Por otro lado, la energía liberada por el ordenamiento parece ser igual a la necesaria para forzar a los cristales a retener su morfología original durante el proceso (19).…”

“…Este estudio pone de manifiesto las limitaciones que tiene el concepto de "fase de Gibbs" en termodinámica en los minerales aquí estudiados, como ya puso Laves de manifiesto en otros feldespatos de tipo Piedra Luna (21). Además tras la existencia de microclinas transicionales subyace la dificultad práctica en la distinción entre "fases posibles" y "fases estables" (5,18). Estas observaciones plantean otros interrogantes: ¿cuanto tiempo tiene que pasar para dejar de considerar "metaestable" a una estructura y considerarla perfectamente estable?…”

Regímenes estático y dinámico en la evolución estructural de microclinas pertíticas

“…298 ºC/Ma (32) no ocurre el ordenamiento de las microclinas desordenadas. Es importante destacar que las microclinas sólo sufren ordenamiento Si/Al si interaccionan con fluidos acuosos que actúan como catalizadores de este proceso (1)(2)(3)(4)(5)11,12). La alcalinidad de los fluidos y la participación de presión durante la catalización de las reacciones del proceso de ordenamiento Si/Al son sus características fundamentales (2,(39)(40)(41).…”