Crystal chemistry of synthetic Mn-bearing anorthite; incorporation of MnAl (sub 2) Si (sub 2) O (sub 8) end-member into feldspar

A series of feldspars with intermediate compositions along the join CaAl (sub 2) Si (sub 2) O (sub 4) -MnAl (sub 2) Si (sub 2) O (sub 4) have been synthesized at 1550 degrees C in an N (sub 2) atmosphere, followed by rapid cooling. Mn may play a role as a mineralizer for megacrystallization of anorthite. The Mn appears to be incorporated into the extra-framework M site of the feldspar structure, with a maximum limit of approximately 25 mol.% MnAl (sub 2) Si (sub 2) O (sub 4) . The Mn-bearing anorthite Ca (sub 0.715) Mn (sub 0.196) Na (sub 0.045<P6M><$B0><P255MV>0.044) Al (sub 1.867) Si (sub 2.133) O (sub 4) has a 8.131, b 12.847, c 7.069 Aa, alpha 94.03, beta 115.89, Gamma 90.72 degrees , V 661.8 Aa (super 3) , space group C<$E 1 back 40 up 40 ->, Z = 4; its structure has been refined to R 3.9% and shows the distribution of Ca and Mn over three split M sites together with disordered Al/Si distribution. Solid solution of the <P6M><$B0><P255D>Si (sub 4) O (sub 8) end-member (the excess silica component) contributes to the shrinkage of the M site making it more favourable for Mn to enter the anorthite structure. The rarity of natural Mn feldspars cannot therefore be attributed to an intrinsic exclusion of Mn from the structure, but to the unusual combination of T, P and chemical conditions required for its crystallization.