Twinning occurs in all kinds of feldspars but is generally not visible with the naked eye. So, although a feldspar crystal appears homogeneous, it may be composed of two or more twin domains.

Most alkali feldspars are compositionally closer to Or end-member which explains why we often refer to them as K-feldspars (potassium feldspars). Microcline is a triclinic K-feldspar and has a characteristic cross-hatched tartan pattern of twinning seen with a petrographic microscope.

Feldspar crystals have a number of important planes. Consequently one may expect a number of different twinning laws to occur. One may classify twins according to the aspects of the twinning laws, for example, according to the combinatory possibilities of planes and axes.

Twinning can have a significant impact on the physical properties of minerals, including their hardness, cleavage, and optical properties. For example, twinned plagioclase feldspar is more resistant to weathering than untwinned plagioclase feldspar.

In general, keys to identifying K-feldspar are its (lack of) color, its low birefringence, and its twinning. In thin section, microcline, orthoclase and sanidine are distinguished by their twinning, optical sign, and 2Vo.

Triclinic System - The feldspar minerals plagioclase and microcline are the most common triclinic minerals that show twinning. Two common twin laws are observed in these feldspars.

feldspar crystal. Description is given in terms of the exes around which twinning takes place, and the composition planes along which the twin Twin crystals formed by simple repetition are represented by the Carlsbad, Baveno, and Manebach twin laws.

The purpose of this chapter is to concentrate on the twinning of feldspars. It has been known for a long time that minerals of the feldspar group are often twinned.

Most alkali feldspars are compositionally closer to Or end-member which explains why we often refer to them as K-feldspars (potassium feldspars). Microcline is a triclinic K-feldspar and has a characteristic cross-hatched tartan pattern of twinning seen with a petrographic microscope.

Twinning in feldspars is most obvious between crossed polars. In addition to simple twins, plagioclase feldspar often shows repeated or multiple twinning (Figures 57c and 59a) usually on a microscopic scale, which gives crystals a striped appearance (Figure 59c).