Description: Let's look again onto the inner face of the right wing (FOV 12x18mm). Guided by typical Kyanite habits, we expect this to be a (100) face, and we expect the wing to be extended along its c[001] axis. (I've also considered the possibility that this might be a (010) face, but this didn't lead to a plausible solution in the end.) The precise direction of the c axis isn't particularly easy to identify in this picture, but the hinge line forms an angle of about 35° with it. Using the known axis ratios and angles for Kyanite, this is consistent with the hinge line being either [012] or [0 1 -2]. (The angle alpha between b[010] and c[001] is almost exactly a right one for Kyanite, which simplifies the calculations, but which results in this ambiguity: it is not possible to pin down the signs of the second and third indices by measuring angles in the (100) plane alone.) The hypothetical twinning/composition plane must contain the hinge line, thus its indices must be of the form (+-h +2l -l) or (+-h +2l +l).

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