Note 100 — Chapter IV · The Microscopic and Optical Characters of Minerals.
Reading edition
By heating feldspar crystals the axial angle decreases to 0° and then increases in the plane of symmetry (at right angles to its former position). On cooling the axial angle returns to its former position if the temperature has not exceeded 500° C. If the temperature has been 600°–1000° C. for some time the axial angle will not return to its former position. This fact may give some clew as to the temperature at which the feldspar crystals formed.