Ion microprobe analyses of exsolution lamellae in peristerites and cryptoperthites

Combined data from electron microscope, electron microprobe, and ion microprobe analyzers can be used to determine the composition and thickness of lamellae in peristerite and cryptoperthite feldspars. Blue iridescent peristerite from Hybla, Ontario has a bulk composition of An₅Or₁, and lamellae compositions of An₂Or₁ and An₁₉Or₁. Orange-red iridescent peristerite from the same location has a bulk composition of An₇Or₁, and lamellae of An₂Or₁ and An₂₃Or₃. Two pale blue iridescent cryptoperthites from Quebec and Johana, Japan have the following compositions: An₁₁Or₃₀ (bulk) and An₁₉Or₁₅, An₀Or₅₁ for lamellae; An₁Or₅₀ (bulk) and An₆Or₃₅, An₀Or₇₂ for lamellae. The irregular nature of the lamellae in peristerites and cryptoperthites precludes ion microprobe depth profiling, such as has been applied to more regular labradorite lamellae. Point analysis must be performed instead, which may result in possible overlap onto adjacent lamellae the extreme limits.