{_ao_ = 100 : 111 7 53 59 − 54 10 54 3 53 54 {_om_ = 111 : 011 7 18 20 − 18 35 18 26 18 29 {_ma_ = 011 : ̄100 13 107 22 − 108 24 107 41 107 37 {_an_ = ̄100 : ̄2̄11 11 62 32 − 63 44 63 6 63 10 {_ns_ = ̄2̄11 : ̄1̄11 1 − 21 35 21 27 {_sa_ = ̄1̄11 : 100 3 94 49 − 95 34 95 18 95 23 {_po_ = 110 : 111 9 43 51 − 44 44 44 35 44 38 {_oc_ = 111 : 001 9 32 56 − 33 15 33 7 33 7 {_cs_ = 001 : ̄1̄11 7 41 32 − 43 2 42 10 42 17 {_sp_ = ̄1̄11 : ̄1̄10 7 59 15 − 60 54 59 59 59 58 {_pc_ = ̄1̄10 : 00̄1 14 77 2 − 78 21 77 42 77 45 {_cp_ = 00̄1 : 110 16 101 39 − 103 36 102 16 102 15 {_pm_ = 110 : 011 9 52 15 − 53 25 52 42 52 41 {_mn_ = 011 : ̄2̄11 5 79 5 − 79 26 79 15 79 12 {_np_ = ̄2̄11 : ̄1̄10 5 47 41 − 48 23 48 4 48 7 {_pr′_ = 110 : ̄201 14 106 35 − 108 42 108 5 108 1 {_r′p_ = ̄201 : ̄1̄10 26 70 54 − 73 32 71 55 71 59 {_ps_ = ̄1̄10 : 1̄1̄1 5 66 42 − 67 17 67 4 67 5 {_sr′_ = 1̄1̄1 : 20̄1 6 40 36 − 41 29 41 5 40 56 _pm_ = ̄110 : 011 3 75 5 − 76 21 75 37 75 45 There is a moderately good cleavage parallel to the basal plane _c_ = {001}. The optical properties afford conclusive proof of the monoclinic nature of the symmetry. The plane of the optic axes is perpendicular to the possible symmetry plane, _b_ = {010}, and the first median line makes an angle of 23° with the vertical axis c, emerging consequently nearly normal to the basal plane _c_ = {001}, so that a section-plate parallel to the _c_-faces, or a tabular crystal or cleavage plate parallel to _c_, shows the optic axial rings and brushes well. The values of the apparent optic axial angle in air, 2E, and of the true optic axial angle within the crystal, 2V_{a}, the latter measured with the aid of a pair of accurately ground section-plates perpendicular to the first and second median lines and immersed in oil, are given in the next table. 2E 2V_{a} For lithium light 51° 35′ 34° 56′ „ sodium „ 52° 30′ 35° 28′ „ thallium „ 53° 50′ 36° 16′ The intermediate refractive index β was found to be as under— For red lithium light 1.4496 „ yellow sodium „ 1.4521 „ green thallium „ 1.4545 The double refraction was also determined and found to be of positive sign.