The phase relations of low quartz, high quartz, and coesite were reinvestigated using extremely pure SiO2. The study was conducted with a piston-cylinder apparatus employing low-friction cells. While the low quartz-coesite transition between 600¿ and 1100¿C was determined by conventional quench techniques, the greater portion of this study relied on in situ methods (differential thermal analysis and a volumetric method referred to as differential pressure analysis). The agreement with previous results is good; however, the precision of present data is considerably higher. A comparison of our salt-cell data with the raw data of previous studies shows a pressure loss due to friction of about 10% for talc cells. A discussion of thermochemical aspects of the low quartz-coesite transition indicates improved agreement between thermodynamic and experimental data. Reconnaissance data on the influence of the NaAlO2 component show about 600¿C in the absence of H2O a considerable shift of the quartz-coesite boundary toward higher pressure (4 kbar at 1100¿C). In the presence of H2O a partial liquid is formed, and the shift is reversed; the transition boundary is reduced 3 kbar and lies between 680¿ and 1000¿ at even lower pressure than that of the pure SiO2 modifications.