  Alkaline earth silicate glasses $xCaO-(1-x)SiO_2$ exhibit a well marked
threshold in Raman lineshapes which can be related to the onset of network
rigidity as the concentration of calcium oxide $x$ is decreased. The present
results are analyzed by constraint counting algorithms and more deeply
characterized by a size increasing cluster approximation that allows to perform
Maxwell mechanical constraint counting beyond the usual mean-field treatment.
This permits to discuss under which structural conditions an elastic
intermediate phase can be obtained.
