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THE FELDSPAR GROUP


The feldspar group is a fairly large group with nearly 20 members recognized, but only nine are well known and common. Those few, however, make up the greatest percentage of minerals found in the Earth's crust. The following are some of the more common feldspar minerals:

The plagioclase feldspars:

The K-feldspars or alkali felspars:

The feldspars are a group of minerals that have similar characteristics due to a similar structure. All feldspars have low symmetry, being only monoclinic, 2/m, 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 and/or potassium, K and/or 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.

The silicons and aluminums occupy the centers of interlinked tetrahedrons of SiO4 and AlO4. These tetrahedrons connect at each corner to other tetrahedrons forming an intricate, three dimensional, negatively charged framework. The cations that represent the X in the formula sit within the voids in this structure.

The different feldspars are distinguished by structure and chemistry. The potassium or K-feldspars are polymorphs, meaning they have the same chemistry, KAlSi3 O8 , but different structures and therefore are different minerals. The plagioclase feldspars are a set of minerals that are in a series from a sodium rich end member, albite, to a potassium rich end member, anorthite. The intermediate members of the series are given arbitrary boundries based on their percentage of sodium or calcium.

Often, feldspars are simply referred to as plagioclase and orthoclase (a K-feldspar) because identification to greater precision is difficult with ordinary methods. Once identified, however, some feldspar mineral varieties are found to have distinctive characteristics or originate from a classic locality and on these bases are recognized by mineral collectors as belonging to a specific feldspar mineral.

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