Purpose of mineral analysis
Among the most interesting aspects of minerals specimens is the presence of crystals. Some collectors may appreciate minerals for their form and color alone, while others may desire to understand the nano-scale properties of the minerals themselves. Some minerals may look quite similar, but have entirely different chemical compositions and crystal structures.
On this web site, many rare and unusual minerals are offered. While it may be desirable to confirm the identity of each sample, practical concerns of finance and time become significant. For many of the samples, Raman spectroscopy and X-ray diffraction have been employed to determine the identity of the mineral species.
At present the functional definition of a mineral consists of a unit cell, and its content of atoms. This implies a chemistry, usually with integral stoichiometry, and a certain symmetry to any crystal faces present. In many cases, determining the size of the unit cell is sufficient to uniquely identify a species. In other cases, a simple chemical test is sufficient. For the most certainty, both a structural study and a detailed chemical analysis are necessary.
Powder X-ray diffraction
When a sample is available in quantity, a powder of the mineral (perhaps the volume of a grain of rice) is made. From this powder, a quick scan of sample can provide a pattern of peaks. The location of the peaks is a consequence of the unit cell dimensions, which is unique, or nearly so, for minerals.
Raman spectroscopy provides a curious mix of structural and chemical information. A laser impinges a sample, which absorbs the laser light, and re-emits light shifted from the laser excitation wavelength. The peaks on a Raman spectrum correspond to energetic modes of vibrations of specific bonds in a mineral. Certain peaks at higher energy are characteristic of hydroxyl, water, borate, carbonate, etc. The number of peaks is quite significant, and quickly allows the distinction of polymorphous minerals (i.e. orpiment and anorpiment).
Unit cell determination
The unit cell is a measurable repeating unit of structure in a crystal. Mathematically, it is a parallelepiped having the symmetry of the whole crystal. While two minerals may have the same crystal structure, subtle differences in bond length are sufficient to distinguish the two separate unit cells. Petrologists routinely measure the unit cell dimensions of olivine are routinely used to quantify the amount of Fe present in forsterite (Mg2SiO4), and more generally separate from fayalite (Fe2SiO4).