Understanding the distribution of mineral compositions is an important challenge in geology. This motivates new variations on dimension reduction for compositional data, lying on a simplex, that approximate observations with lower-dimensional subsimplices. This representation interprets compositions as convex combinations of fewer number of latent source compositions, providing unique insight into the underlying data-generating processes. We develop this approach within the framework of backwards principal component analysis, which constructs nested approximating structures by successive dimension reduction by rank one. We introduce a class of subsimplices, termed proper subsimplices, and provide a formal definition together with a systematic study of their geometric properties. Proper subsimplices admit effective parametrization via hyperplanes, while providing a more flexible approximation than subsimplices obtained by amalgamation. We characterize simple conditions under which hyperplanes generate proper subsimplices. Based on these conditions, we propose both an exact algorithm and a computationally efficient approximate algorithm for estimating proper subsimplex representations. The usefulness of this method is demonstrated on two geochemical datasets where it gives improved insights compared to the conventional PCA approach. Main ideas are further illustrated in a toy example and a simulation study.

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