Paul, have you considered explicitly multiple scattering from within the tetrahedron?  The short distance you describe for the second shell might be consistent with that.

Its pretty easy to write an expression in the fit routine that makes the MS path distances explicitly dependent on the SS paths (no new variables in the fit)--the number of MS paths is constrained by the geometry (tetrahedral).  The order of scattering is probably just 3- and 4- legged MS paths, no higher (would give longer distances)

I study arsenic, and so I've seen this approach described in the following paper on As:  

Voegelin et al (2007) Geochimica et Cosmochimica Acta v. 71, pp. 5804-5820.  Look in the section 2.6 for the details.

sorry if someone already answered this,


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Andrea L. Foster, Ph.D.
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