Can a reaction be written between a set of minerals within error?

Determining whether a reaction can be written between a set of minerals wit hin error is an example of the more general problem of determining whether a set of compositions are coplanar within error. Generally if the compositi ons are of minerals, 'within error' should allow the minerals to maintain s toichiometry. The problem is addressed via the addition of a 'bogus' compos ition to the set, and calculating the reaction coefficients for a reaction between the compositions in this augmented set. A reaction can be written i f a confidence interval on the reaction coefficient of 'bogus' includes zer o. The reaction coefficients can be solved for directly when the problem is posed in terms of least squares with equality constraints. The confidence interval is determined with a bootstrap method, allowing the result to depe nd on the scatter of the data around the solution of the least squares prob lem, not on the data uncertainties.