Triclinic feldspars: angular relations and the representation of feldspar series

The rhombic section angles and obliquities of tricl inic feldspars are simple trigonometric functions of a * and 'y. Alkali-exchange series and short-term heating experiments how wide variation in a * but l i tt le variation in 7. Differing degrees of Al-Si order atconstant composi-tion and temperature cause wide variations in 7 and relatively minor ones in a*. Graphical methods are developed by which a*,,y, rhombic section angle, and obliquity can be read easily from a single plot. The same plot also differentiates the various types of feldspar series at least as well as any other in current use. The obliquity, @, is a direct measure of the departure of a tricl inic feldspar from the dimensional requirements for monoclinic symmetry. The quantity (l- cos@) must vanish for monoclinic feldspars and is a nearly l inear function of temperature for isochemical feldspar series that approach a monoclinic state on heating. A new convention is proposed for defining the rhombic section of potassic feldspars in order to make their values consistent with those of contiguous K-Na feldspars. Feldspars of a given composition that are in full internal equil ibrium should l ie along unique paths in terms of a * and 7. Sufficient information is now at hand to show the general form of such paths lor K-Na-Ca feldspars, and to provide preliminary formulations of these paths for the end-member feldspars