Elements in the same groups (columns) have their outermost electrons in the same kind of orbitals. This is very significant because it means the elements have similar chemical properties, form similarly charged ions and, importantly, commonly substitute for each other in mineral crystals. For example, the mineral specimen shown in Figure 2.4 is an alkali feldspar. When it first crystallized from a magma, it was a homogeneous mixture of Na-feldspar and K-feldspar. Na and K, both alkali elements (Group 1), commonly substitute for each other in minerals. When this sample cooled, however, the two different feldspar components unmixed from each other, much the same way that chicken soup and fat separate on cooling. The result is called exsolution (which means unmixing) and the specimen now contains thin pinkish veins of K-feldspar surrounded by Na-feldspar. We call feldspar that has exsolved like this, perthite.
2.4 Perthite (exsolved alkali feldspar)