Described in this study is the correction of crystallographic axes for crystal form drawings for C2 /m amphiboles, along with an outline of methodology and updates to the spreadsheet EXCELIBR. This error may perpetuate a misunderstanding between the crystallographic setting and optical orientation of clinoamphiboles, which is an important relationship for orientation-dependent analytical methods. In the correct orientation of a typical C2 /m amphibole, the physical optical orientation should have never changed from its position outlined in the Tschermak setting as shown in Ford and Dana (1932), however, the crystallographic axes and ( hkl) should have changed to accommodate the difference between the I2 /m cell and the C2 /m cell. Using the methods outlined by Gunter and Twamley (2001) combined with X-ray and optical methods on single crystals of amphiboles reveals the discrepancy between axes. When C2 /m became the standard space group, the optical orientation ( hkl), and crystallographic axes depicted in crystal form drawings were never revised. Texts citing the early optical work on amphiboles reference structures drawn in the I2 /m cell, for which the optical orientation is correct. The crystallographic orientation of C2 /m amphiboles has been depicted incorrectly since the standardization of amphiboles in C2 /m.

Crystalmaker northwestern