Summary: A sulphatic cancrinite, with 5·93 % SO3, is described from Loch Borolan, Assynt, Scotland, and compared with three other sulphatic cancrinites previously described, and with normal cancrinites. They form an isomorphous series, in which SO3 replaces CO2, and the sulphatic end-member has not yet been recorded. The influence of the SO3 content on the optical properties is discussed. It is shown that the higher refractive index (ω) and the birefringence decrease fairly rapidly with increasing SO3. The mineral probably changes from negative to isotropic and then to positive, with increasing SO3 content.
A birefringent variety of analcime occurs in a pegmatitic patch in the borolanite, Loch Borolan. It is optically negative and usually biaxial. It shows twin-lamellae parallel to the cubic cleavages, and occasional subordinate dodecahedral lamellae. The optical anomalies are very similar to those of eudnophite, and it is suggested that they may be due to strain during crystallization.