Revision 1 1 2 EXCALIBR to EXCELIBR and the Optical Orientation of Minerals : Correcting the Optical Orientation of 3 Clinoamphiboles . 4 5

9 10 The crystallographic orientation of C 2/m amphiboles has been depicted incorrectly since the 11 standardization of amphiboles in C 2/m. Texts citing the early optical work on amphiboles reference structures 12 drawn in the I 2/m cell, for which the optical orientation is correct. When C 2/m became the standard space group, 13 the optical orientation, (hkl), and crystallographic axes depicted in crystal form drawings were never revised. Using 14 the methods outlined by Gunter and Twamley (2001) combined X-ray and optical methods on single crystals of 15 amphiboles reveal the discrepancy between axes. In the correct orientation of a typical C 2/m amphibole, the 16 physical optical orientation should have never changed from its position outlined in the Tschermak setting as shown 17 in Ford and Dana (1932), however, the crystallographic axes and (hkl) should have changed to accommodate the 18 difference between the I 2/m cell and the C 2/m cell. This error may perpetuate a misunderstanding between the 19 crystallographic setting and optical orientation of clinoamphiboles, which is an important relationship for 20 orientation-dependent analytical methods. Enclosed in this study is the correction of crystallographic axes for crystal 21 form drawings for C 2/m amphiboles, along with an outline of methodology and updates to the spreadsheet 22 EXCELIBR. The methods applied in this study utilize relationships between crystallographic and optical vectors and 23 include an addendum to those presented by Gunter and Twamley (2001), which is applicable to arbitrary reference 24 positions on spindle stages. 25