FELDSPAR

Similar dense-packed tetrahedra characterize the crystal framework of feldspar minerals, as depicted in Figure 4.

Feldspar

 
This figure shows a single layer viewed perpendicular to its plane. The framework is extended by rotating each successive layer 90 degree. Feldspars usually have one of every four Si4+ substituted with Al3+. The resulting charge imbalance is compensated by sodium, and/or potassium ions. Some feldspars have half their silicon replaced by aluminum with calcium balancing the framework charge. Feldspars are nearly as hard as quartz and are exploited for their chemistry in glassmaking and ceramics, since the aluminum content improves chemical and physical stability while its alkali content provides fluxing action.