EXCALIBR to EXCELIBR and the optical orientation of minerals: Correcting the optical orientation of clinoamphiboles

The crystallographic orientation of C2/m amphiboles has been depicted incorrectly since the standard‑ ization of amphiboles in C2/m. Texts citing the early optical work on amphiboles reference structures drawn in the I2/m cell, for which the optical orientation is correct. When C2/m became the standard space group, the optical orientation (hkl), and crystallographic axes depicted in crystal form drawings were never revised. 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. In the cor‑ rect 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. 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. 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. The methods applied in this study utilize relationships between crystallographic and optical vectors and include an addendum to those presented by Gunter and Twamley (2001), which is applicable to arbitrary reference positions on spindle stages.