For a half-century, mineralogists and petrologists have been splitting up bulk chemical analyses of minerals into the parts belonging to the individual minerals. If minerals had simple compositions with no replacement of atoms, such an element assignment would be easy. However, the majority of minerals have complex formulae in which several elements join together to occupy the atomic sites. In a typical mineral assemblage, several of the elements will be shared by two or more of the minerals. The elemental assignment in a mineral assemblage, therefore, may present some pretty problems to the mineralogist, and as a result the mineralogists have attained some skill in dealing with such problems. As a result of being able to control the composition of their substances to a fair precision, many chemists are unaware of the mineralogists' procedures for dealing with these complex compositions, procedures that have considerable value in the study of water-formed deposits. A description of these procedures should be of value to those who have not received a mineralogical training. Water-formed deposits, of course, have their analogy in those natural minerals formed in fissures of the earth by the prolonged streaming of mineral-laden solutions. Indeed, nearly all of the show specimens seen in the mineralogical museums have been formed by deposition from solutions. Rules which can be used to predict the possible chemical substitutions in minerals will be described, then some phase-equilibrium diagrams will be given to show the effect of temperature on atomic substitution. Following a discussion of the physical methods that can be used to estimate the volume percentage and chemical composition of individual minerals in an inseparable assemblage, six bulk chemical analyses will be examined to see whether and how the elements may be assigned to the individual minerals.