What is the partial molar volume of epsomite (magnesium sulfate heptahydrate)? I am trying to calculate the expected volume of an aqueous solution of epsomite and NaCl (308 deg K but willing to go with 298 K as close enough, and 1 atmosphere pressure).  I realize the two may interact, but as a first approximation I'm looking for the partial molar volumes of both. I found NaCl's in a paper (Archer (1992) J Phys Chem Ref Data 21(4): 793-829), but I can't find the values for epsomite.

I have a known quantity of salts and am trying to figure out how much solution I'll end up with after adding enough water to get a particular overall specific gravity. For the curious, I've got 600 pounds of epsomite, 50 pounds of NaCl, and I'm shooting for a specific gravity of 1.25.  Knowing the amount of water to add is straightforward and, unfortunately, not sufficient for this particular situation. I really need to calculate the total final volume after the salts have dissolved.

Thank you.
As you know, partial molar volume of given substance is the change in solution volume after one mole of this substance is added. When you add $\ce{MgSO4.7H2O}$ to water it dissolves to magnesium cation and sulfate anion but the hydration water simply mixes with the rest of water. So, I suppose that partial molar volume of the magnesium sulfate heptahydrate is the sum of partial molar volume of magnesium sulfate and seven times molar volume of water. If $\widehat{V}_i$ denotes partial molar volume of substance $i$, and $V_i$ denotes molar volume, then: $$\widehat{V}_{\ce{MgSO4.7H2O}}=\widehat{V}_{\ce{MgSO4}}+7V_{\ce{H2O}}.$$
Nevertheless, partial molar volume is function of composition and commonly quoted values are for very low concentration (adding substance to large quantity of water). If you want to obtain solution with higher concentration, finding its exact volume requires knowledge about $\widehat{V}_i$ as a function of composition and such function has to be integrated.