Feldspar laths

The above-mentioned organic acids may be expected to produce conditions in which silica of silicate minerals is dissolved, and — assuming the phosphate concentration is adequate or sufficient time has elapsed at lower concentrations — the silicate minerals become replaced by phosphate minerals. Although little is known concerning the conditions of oxidation vs. reduction encountered in guano deposits, such a mode of replacement must be attributed to the conversion of feldspar laths to variscite-metavariscite at Malpelo Island (McConnell, 1943). Indeed, the conditions may be aerobic at the surface, whereas anaerobic conditions obtain at the lower portions in contact with the country rock. 

The ternary compound, potash feldspar, has an incongruent melting point at 1150 + 20°C. In all compositions that lie in this field, considerable difficulty is experienced in obtaining potash feldspar crystals. Even after several months of crystallization at temperatures about 50 to 75°C below liquidus temperature, Schairer and Bowen [75] observed only a few percent of very small feldspar laths in the glasses. For those extremely viscous melts that also lie on or very near the leucite-silica line, Schairer and Bowen never crystallized feldspar. 

The granite outcrops are medium- to coarse-grained and have a hypidio-morphic granular texture dominated by rounded or interlocking quartz grains, and laths of pink, white, and perthitic feldspars. 

When firing a mixture of kaolinite and feldspar, the first traces of mulUte are obtained at about 1000 °C. The onset of transformation can be detected in kaoUnite single crystal relics, where the formation of crystallographically oriented muUite laths is aided by the presence of potassium ions. 

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