The microline-orthoclase controversy -- can microline be primary?

A common perception among petrologists has persisted that microcline is always derived from orthoclase (a high-temperature mineral), and, therefore, all microcline-bearing granites of large dimension have crystallized from a magma. This perception is based on studies of Laves (1950) who suggested that the gridiron twinning in microcline can form only by inversion from the high-temperature lattice in orthoclase because of the peculiar relations between pericline and albite twinning in microcline; see also MacKenzie (1954). In magmatic rocks, primary orthoclase will under certain conditions invert to secondary microcline because of the ordering of (Si, Al) atoms during the unmixing of Na and K phases to form perthite. This inversion is quite sluggish, however, and rarely, can a pluton be found, if ever, in which gradations occur from unaltered orthoclase to places where all orthoclase is completely inverted to microcline. Complete gradations occur, however, in some Precambrian granulites, but in these rocks deformation and shifting to lower grade metamorphic conditions (lower T-P) likely aids the total inversion of orthoclase to microcline (Eskola, 1952). Nevertheless, microcline does not always have to result from an orthoclase inversion. Considerable geologic, textural, and temperature evidence exists, demonstrating that microcline can form as a primary mineral at temperatures below the solidus where there is absolutely no support for the hypothesis that orthoclase was first formed at higher temperatures as a precursor to this microcline