The low-temperature phases of the alkali feldspars and their origin

New relationships among the low-temperature alkali feldspars were deduced in part and implied in whole by recent crystal structure analyses of low and high and consideration of the electrostatic charge distributions in all the known alkali feldspar structures by Ferguson, Traill & Taylor. In general the most stable low-temperature forms are not those most highly ordered with respect to Al-Si as suggested by Goldsmith & Laves, but those most highly balanced electrostatically: slightly ordered orthoclase and largely ordered low albite. Because of the mobility of the alkali atoms at higher temperatures, appreciable amounts of Na in a cooling K-Na feldspar tend to make the Al-Si framework triclinic so that, after exsolution, the K component is triclinic microcline. There thus exists a series of low-temperature K feldspars varying from orthoclase formed in the presence of relatively little Na, through microcline to microcline formed in the presence of considerable excess of Na over K. It is shown that the (dimensional) monoclinic to triclinic change in the low-temperature K feldspars occurs at an original alkali feldspar composition of about 20% NaAlSi 3 O 8 . Intermediate microclines very similar to the low-temperature forms may also result from processes involving rapid cooling from temperatures of K feldspars in which Na again played an earlier essential role. The theory also predicts an analogous series of low-temperature Na feldspars varying from ordinary low-temperature formed in the presence of relatively little K, through low-temperature intermediate to a maximum albite formed in the presence of considerable excess of K over Na. Evidence for the existence of such a series is discussed. Intermediate albites formed by rapid cooling from temperatures apparently also exist. The ionic picture of the alkali feldspars thus leads to the following low-temperature phases on the phase diagram: orthoclase, orthoclase + albite, microcline + albite, microcline + low albite, low albite. Some of the many implications of the theory are discussed.