Summary: Hydrothermal almost pure potassium feldspar (adularia) forms incrustations on fissured wall rocks in an active geothermal environment. Measured max. temperature, 265 °C, and concentration of K+ and Na+ cations in the geothermal fluid require K-feldspar and Na-feldspar to be deposited under equilibrium conditions. On fissure walls only K-feldspar is precipitated, whereas andesine phenocrysts of host rocks are replaced by both K-feldspar and Na-feldspar, and groundmass of host rock is replaced by K-feldspar.
Optical data indicate that K-feldspar forming incrustations is partly monoclinic, partly triclinic. The triclinic material displays polysynthetic and ‘microcline’-like twinning, though X-ray diffractograms indicate only monoclinic structure. A primary origin for the triclinic adularia is postulated. Wairakei hydrothermal K-feldspar, max. 0·5 Myr old, is compared with Arkansas adularia 287 or 214 Myr old, also containing both monoclinic and triclinic material. The comparison suggests that, after crystallization, the structural state of adularia changes little, if at all, during geological time.