Refinement of the crystal structure of a ternary barium feldspar with the formula (Ba0.40K0.47Na0.13)(Si2.59Al1.41)O8, a = 8.544(2), b = 13.030(2), c = 7.195(2)Å, β = 115.68(2)°, space group = C2/m yields a disordered Al,Si distribution, with 0.437 Al in the T1 site and 0.268 Al in the T2 site. On the other hand, the lattice parameter c suggests that the investigated specimen probably has a structural state very close to that of maximum order possible for its composition. It is therefore concluded that the state of maximum order for a natural (Ba,K)-feldspar with a Cn (celsian) content of 40 mol% corresponds to a disordered Al/Si distribution. Comparison with the structural data of a Ba-poor feldspar indicates that the disorder shown by such maximum ordered (Ba,K)-feldspars increases with their Cn content.

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