Most published analyses of garnet deviate from structural ideality. Consequently, compositions expressed as molecular percentages of end-member molecules may differ if different re-calculation sequences have been used. A suitable standard calculation procedure is presented, and is demonstrated to be satisfactory by its application to 69 published analyses of garnets both common and rare.
It is seldom necessary to use molecules other than pyrope, almandine, spessartine, grossular, andradite, uvarovite and hydrogrossular, and most analyses can be recast into four or less molecules which exceed 3% of the garnet. This means that most analyses can be visually displayed in a composition tetrahedron.
It is suggested, that the percentage number of cations which can be allocated to garnet molecules is a figure useful for assessment of analytical quality. More than 95% of the cations can be so allocated in the majority of the analyses considered.
Full details of the proposed scheme are appended together with a worked example which demonstrates the abbreviated procedure which applies to most common garnets. A compilation is given of the common physical properties which have been measured for synthetic end-member garnets of the types used in the calculation scheme.
S.A. UMP Publication No. 5.