Intermediate alkali feldspars

Intermediate Alkali Feldspars. The alkali feldspars with intermediate composition vary in their TL signal when seen in detail. All of them differ significantly as a group from the end members. However, several interesting common features throw light on their overall behaviour. These features suggest a new means for identifying structure, which differs from the traditional optical and X-ray diffraction methods. 

Many minerals show exsolution textures. For these minerals, there is complete solid solution at sufficiently high temperatures, but a miscibility gap at lower temperatures. The alkali feldspar, (Na, K)AlSi30s, is an example. Suppose a roughly homogenous mineral of intermediate composition cools down and into... 

Effect of pH. Most minerals, including carbonate minerals (Wollast 1990), exhibit a transition from pH-independent to pH-dependent dissolution (Fig. 5). The critical question is whether the pH of the transition is above or below the pH of the soil solution. The transition points for a number of minerals are shown in Table 2. If alkali feldspars and intermediate plagioclases are the most important sources of cations in weathering (Garrels and Mackenzie 1967), then the transition pH will be about 4.5, and the weathering rate will increase as What is a reasonable pH range for... 

In this section, we must consider the alkali feldspars either low or high in K, in low-, intermediate-, and high-temperature forms the plagioclase feldspars, in low- and high-temperature forms, and at several points in the composition range between albite and anorthite and the barium feldspars. 

Reported assemblages in sedimentary rocks containing analcite and potassic feldspar plus an alkali zeolite (Hay, 1966 Sheppard and Gude, 1968, 1969 Moiola, 1970) lead to the construction of an intermediate paragenesis which denotes a more restricted range of alkali zeolite solid solution, notably the instability of the alkali-rich phase phillipsite. 

Due to the conflicting experimental results, we will rely upon natural occurrence and we will place an 80°C limit on the initial zeolite paragenesis where solid solution is maximum. An intermediate stage exists up to temperatures near 100°C at high water pressure where analcite, potassium feldspar and alkali zeolite can coexist as can analcite and albite. Analcite persists up to 180°C where the upper limit of the alkali zeolite facies is reached. 

---