© 2018 Elsevier B.V. The authors regret that Fig 3 was incorrectly printed as a duplicate of Fig. 4. The correct Fig 3 with original caption is given here. The authors would like to apologise for any inconvenience caused. [figure-presented] Caption Fig. 3. Plot of H against ϕ i using Eq. (7), with ϕ o = 0.20. H = 0 when ϕ i = 0.20. When F = 0 there are no inclusions and thus <ϕ m > and ϕ o will be the same (since there is no ϕ i ). At other values of ϕ i , H ≠ 0, and there could be error. At ϕ i = 0.20, H = 0 for all the F's. At H= 0.1, the dashed line, the difference between <ϕ m > and ϕ o will be ∼10%, but below this value the assumption of core homogeneity may probably create little extra uncertainty.