Isomorphic substitutions in afghanite: Identification of the mineral

Samples of afghanite, (Na,Ca,K)₈ (Al₆Si₆O₂₄)(SO₄,Cl,CO₃)₃ · H₂O, from the Malaya Bystraya and Tultui lazurite deposits in the Baikal region have been studied with X-ray diffraction and an electron microprobe. The unit-cell dimensions a and c of the examined afghanite samples range from 12.729 to 12.762 &0A and from 21.385 to 21.415 &0A. Isomorphic substitution is exhibited between Na and K; Ca content is about 2.5 apfu; the Cl/SO₄ ratio is close to 1.0 with slight variations. Parameter a was plotted versus K content. The correlation between Na and K is established. Comparative analysis has shown that afghanite from the Baikal region is characterized by a lower K content than that from volcanic complexes in Italy. In the studied samples, K₂O ranges from 0.51 to 2.28 wt %, whereas in comparable samples from volcanic complexes of Italy, it varies from 3.44 to 8.26 wt %. Sodalite-group minerals display a similar behavior of potassium. In lazurite from the Baikal region, the K₂O content ranges from 0 to 0.95 wt %; the K₂O content in hauyne from Italy varies from 0.88 to 5.75 wt %. The wider isomorphic miscibility in the feldspar minerals from volcanic-hosted assemblages in Italy as compared with the same minerals from lazurite deposits of the Baikal region reflects the difference in the geological and physicochemical conditions under which the minerals have been formed. In addition to the X-ray procedure, the Cl/SO₄ ratio determined from microprobe data may be used to identify cancrinite-group minerals found at the Baikal lazurite deposits. It is near 1.0 in afghanite; 0.8 in tounkite; and more than 2 in davyne.