Abstract
To facilitate systematic study of the surviving compositionally pristine (endogenously igneous) rocks of the ancient lunar crust, a compilation has been generated of all likely samples, along with key information on the petrologic characteristics and chemistry of each sample. The compilation includes 260 samples. Besides information related to the likelihood of each sample being truly pristine (i.e., mainly its texture and siderophile element abundances), information is compiled on mineral content, listing major phases present as well as basic information on mineral compositions, on size (expressed as mass), and on whether a reasonably comprehensive chemical analysis has been published. The compilation also classifies the samples into seven categories of confidence in the pristine composition of the samples, reflecting an estimation of the relative likelihood that each arguably pristine sample is in fact pristine. For many purposes, it is crucial to avoid inclusion of polymict rocks in a data base. On petrologic diagrams such as a plot of average Mg′ [Mg′ = 100 × molar Mg/(Mg + Fe)] in a low-Ca mafic silicate vs. average An content in plagioclase, rocks in the top three categories of the confidence in pristine character appear distinctly bimodal in composition, with roughly half belonging to a ferroan suite characterized by high An despite relatively low Mg′. When samples of low to moderate pristine character are included, the bimodality appears less distinct. Sample mass can also be important. With a data base restricted to samples more massive than 1 g, there is a clear distinction in feldspar content and bulk density between ferroan and nonferroan (Mg-suite) rocks, such that only the ferroan-suite rocks are likely to have formed as flotation cumulates. With a data base including smaller samples, the same basic pattern is seen, but only in a blurred form, as the two rock types show considerable overlap in their modal feldspar contents.
