How can we determine how a mineral is bonded from its chemical formula?

Question

Take Orthoclase (KAlSi3O8) for example.From the formula how can we determine the type of bonding involved EG. ionic, covalent...other ?

Answer 1

While x-ray diffraction etc. will tell you the arrangement of atoms in a crystal, it will not tell you what types of bonds hold those atoms together. Fortunately, we can work out what types of bonds from simply considering the kinds of atoms in the mineral. From chemical principles, most metals form ionic bonds, non-metals form covalent bonds, and water forms hydrogen bonds. Simple ions (sulfate, carbonate, etc.) form ionic bonds with metal ions. That's how it works inside crystals, and we can use these principles to work out what types of bonds are present in minerals. So in Orthoclase, we have covalently bonded Si, O and Al (yes, Al is a metal, but in this case it can bond covalently due to it's chemical similarities to Si) forming a 3D framework, with K+ ions distributed throughout, and ionically bonded to, the Si-Al-O structure. In reality, it's a bit more complex: all bonds are partly covalent and partly ionic, and we can decompose the compound under lab conditions and measure the energy needed to get at what percent ionic vs. covalent each type of bond is.

Answer 2

X ray or electron diffraction. Also conventional qualitative chemical analysis.