Potash Feldspar

The feldspars are a group of minerals that have similar characteristics due to a similar structure. All feldspars have low symmetry, being only monoclinic, 2m, to triclinic, bar 1. They tend to twin easily and one crystal can even be multiply twinned on the same plane, producing parallel layers of twinned crystals. They are slightly hard at around 6, and have an average density at 2.55 to 2.76. They have a rather dull to rarely vitreous luster. Crystals tend to be blocky. Some feldspars may be triboluminescent. They have two directions of cleavage at nearly right angles. Feldspars also tend to crystallize in igneous enviroments, but are also present in many metamorphic rocks.
The general formula, for the common feldspars, is xal(1-2) si(3-2) o8 . The x in the formula can be sodium, na andor potassium, k andor calcium, ca. When the cation in the x position has a positive one (+1) charge such as with sodium or potassium, then the formula contains one aluminum and three silicons ions. If the formula contains the positive two (+2) cation calcium, then the formula will contain two aluminums and only two silicon ions. This substitution keeps the formula balanced, because aluminum has a charge of positive three (+3) and silicon has a charge of positive four (+4). Basically, the more calcium in the crystal, the more aluminum that will be needed to balance the charge.