Magnesio-hornblende from Luderitz, Namibia: mineral description and crystal chemistry

Magnesio-hornblende (IMA2017-059) has been characterized in a specimen collected in the sand dunes of Luderitz, Karas Region, Namibia. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement is (A)(square 0.73Na0.22K0.05)(Sigma 1.00)(B)(Ca1.79Fe0.102+Mg0.04Mn0.032+Na0.04)(Sigma 2.00)(C)(Mg3.48Fe0.972+Al0.28Fe0.233+Cr0.013+Ti0.03)(Sigma 5.00)(T)(Si7.18Al0.82)(Sigma 8.02)O-22( W)[(OH)(1.93)F0.005Cl0.22](Sigma 2.00). Magnesio-hornblende is biaxial (-), with alpha = 1.640(2), beta = 1.654(2), gamma = 1.666(2) (measured with gel-filtered Na light, lambda= 589.9 nm), 2V (meas.) = 82(1)degrees and 2V (calc.) = 84.9 degrees. The unit-cell parameters are a = 9.8308(7), b= 18.0659(11), c= 5.2968(4) angstrom, beta = 104.771(6)degrees and V = 909.64 (11) angstrom(3) with Z= 2 and space group C2/m. The strongest eight reflections in the X-ray powder pattern [d values (in angstrom), I, (hkl)] are: 2.709, 100, (151); 8.412, 74, (110); 3.121, 73, (310); 2.541, 58, ((2) over bar 02); 3.386, 49, (131); 2.596, 45, (061); 2.338, 41, ((3) over bar 51); and 2.164, 39, (261).