A method for in situ quantitative microanalysis of exsolution lamellae in labradorite (“Bϕggild intergrowth”) has been developed, using an ion microprobe mass analyzer. Ion probe analyses for two kinds of exsolution lamellae of low-temperature intermediate plagioclases showing various iridescent colors led to the calculation of the following determinative curves:

Mol % An = 97.0[9.4] – 142.1[27.5](INa+/IA1+)corr, where (INa+/IAl+)corr is the observed (INa+/IAl+) multiplied by an An correction factor ƒAn.

Mol % Or = 0.73[0.28] + 104.8[33.0](IK+/IAl+)corr, where (IK+/IAl+)corr is the observed (IK+/IAi+) multiplied by an Or correction factor ƒOr.

From trace analysis of a micron area (“line analysis”) and three-dimensional analysis of lamellar multilayer including the direction of depth (“in-depth analysis”), the mean chemical compositions of An-poor and An-rich lamellae are An44(±4)Or3(±1) and An58(±6)Or2(±1), respectively. The mean composition difference between adjacent lamellae is 15 percent An and 1 percent Or.

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