This investigation reports on a linear stability analysis of the quiescent state within a horizontal porous cavity subject to vertical gradients of temperature and solute. The fluid motion is modeled using the Brinkman extension of Darcy's law, coupled with energy and species conservation equations. The horizontal boundaries are considered rigid-rigid, rigid-free, or free-free. Mixed thermal and solutal boundary conditions, of Dirichlet and Neumann types, are considered. The thresholds for monotonic and oscillatory convection instabilities are determined explicitly in terms of the governing parameters of the problem. The results for a viscous fluid and the Darcy porous medium emerge from the present analysis as limiting cases.