  The geometric reinterpretation of the Finite Element Method (FEM) shows that
Raviart Thomas and Nedelec mass matrices map from degrees of freedoms (DoFs)
attached to geometric elements of a tetrahedral grid to DoFs attached to the
barycentric dual grid. The algebraic inverses of the mass matrices map DoFs
attached to the barycentric dual grid back to DoFs attached to the
corresponding primal tetrahedral grid, but they are of limited practical use
since they are dense.
  In this paper we present a new geometric construction of sparse inverse mass
matrices for arbitrary tetrahedral grids and possibly anisotropic materials,
debunking the conventional wisdom that the barycentric dual grid prohibits a
sparse representation for inverse mass matrices. In particular, we provide a
unified framework for the construction of both edge and face mass matrices and
their sparse inverses. Such a unifying principle relies on novel geometric
reconstruction formulas, from which, according to a well established design
strategy, local mass matrices are constructed as the sum of a consistent and a
stabilization term. A major difference with the approaches proposed so far is
that the consistent term is defined geometrically and explicitly, that is,
without the necessity of computing the inverses of local matrices. This
provides a sensible speedup and an easier implementation. We use these new
sparse inverse mass matrices to discretize a three dimensional Poisson problem,
providing the comparison between the results obtained by various formulations
on a benchmark problem with analytical solution.
