Density Matrices and Geometric Phases for n-state Systems
  An explicit parameterization is given for the density matrices for $n$-state systems. The geometry of the space of pure and mixed states and the entropy of the $n$-state system is discussed. Geometric phases can arise in only specific subspaces of the space of all density matrices. The possibility of obtaining nontrivial abelian and nonabelian geometric phases in these subspaces is discussed. 
Luis J. Boya, Mark Byrd, Mark Mims and E. C. G. Sudarshan