Eclarite Diffraction data computed using the structure from the paper listed below, along with the cell parameters refined from single crystal data of R110124 Kupcik V Tschermaks Mineralogische und Petrographische Mitteilungen 32 (1984) 259-269 Die kristallstruktur des minerals eclarit, (Cu,Fe)Pb9Bi12S28 _database_code_amcsd 0015698 CELL PARAMETERS: 54.7620 4.0297 22.7010 90.000 90.000 90.000 SPACE GROUP: Pnma ATOM X Y Z OCCUPANCY ISO(B) Cu 0.30550 0.75000 0.13900 0.600 3.200 Fe 0.30550 0.75000 0.13900 0.400 3.200 Pb 0.00840 0.25000 0.23460 1.000 2.200 Pb 0.11630 0.75000 0.00350 1.000 2.500 Pb 0.17910 0.75000 0.11770 1.000 2.300 Pb 0.18420 0.75000 0.31580 1.000 2.100 Pb 0.24950 0.75000 0.20650 1.000 1.800 Pb 0.32160 0.25000 0.00040 1.000 3.100 Pb 0.36930 0.25000 0.13820 1.000 3.100 Pb 0.43010 0.25000 0.03400 1.000 3.100 Pb 0.02600 0.25000 0.04450 0.200 3.100 Pb 0.00600 0.25000 0.42110 0.200 1.800 Pb 0.37930 0.75000 0.32740 0.200 1.800 Pb 0.43770 0.75000 0.20540 0.200 1.300 Pb 0.44120 0.25000 0.36740 0.200 1.500 Bi 0.07010 0.75000 0.14810 1.000 1.400 Bi 0.06580 0.75000 0.33080 1.000 1.200 Bi 0.12560 0.25000 0.22210 1.000 1.700 Bi 0.12310 0.25000 0.40920 1.000 1.500 Bi 0.24510 0.25000 0.03570 1.000 1.600 Bi 0.25130 0.25000 0.37540 1.000 2.000 Bi 0.31620 0.25000 0.27980 1.000 1.500 Bi 0.31970 0.75000 0.44260 1.000 1.200 Bi 0.02600 0.25000 0.04450 0.680 3.100 Sb 0.02600 0.25000 0.04450 0.120 3.100 Bi 0.00600 0.25000 0.42110 0.680 1.800 Sb 0.00600 0.25000 0.42110 0.120 1.800 Bi 0.37930 0.75000 0.32740 0.680 1.800 Sb 0.37930 0.75000 0.32740 0.120 1.800 Bi 0.43770 0.75000 0.20540 0.680 1.300 Sb 0.43770 0.75000 0.20540 0.120 1.300 Bi 0.44120 0.25000 0.36740 0.680 1.500 Sb 0.44120 0.25000 0.36740 0.120 1.500 S 0.02230 0.75000 0.13600 1.000 1.100 S 0.01900 0.75000 0.32400 1.000 1.200 S 0.07520 0.25000 0.06700 1.000 1.500 S 0.06450 0.25000 0.24000 1.000 2.100 S 0.06540 0.25000 0.41600 1.000 1.000 S 0.12550 0.75000 0.14000 1.000 1.200 S 0.12510 0.75000 0.31600 1.000 1.800 S 0.12300 0.75000 0.48900 1.000 1.900 S 0.15320 0.25000 0.03600 1.000 0.600 S 0.17250 0.25000 0.21800 1.000 1.300 S 0.17080 0.25000 0.40900 1.000 2.000 S 0.22130 0.25000 0.13600 1.000 1.300 S 0.22580 0.25000 0.28400 1.000 0.400 S 0.22360 0.75000 0.41800 1.000 1.100 S 0.27500 0.75000 0.06200 1.000 1.200 S 0.29280 0.25000 0.18300 1.000 1.000 S 0.28530 0.75000 0.32300 1.000 1.300 S 0.29120 0.25000 0.48700 1.000 1.400 S 0.34010 0.75000 0.08000 1.000 1.400 S 0.34680 0.75000 0.23200 1.000 2.400 S 0.35320 0.25000 0.39400 1.000 1.500 S 0.40820 0.75000 0.10400 1.000 2.100 S 0.40510 0.25000 0.26900 1.000 1.200 S 0.41110 0.75000 0.42000 1.000 2.900 S 0.47580 0.75000 0.03500 1.000 2.300 S 0.46460 0.25000 0.16400 1.000 1.600 S 0.46630 0.75000 0.30700 1.000 1.500 S 0.47330 0.25000 0.46200 1.000 2.900 X-RAY WAVELENGTH: 1.541838 BOUNDS ON TWO THETA: 5.0 90.0 LIMITS IMPOSED ON THE INDICES ARE: +/- 50 +/- 3 +/- 20 MAX. ABS. INTENSITY / VOLUME**2: 49.02818268 The INTENSITY cut off value is 1.00 2-THETA INTENSITY D-SPACING H K L 5.06 1.29 17.4759 2 0 1 6.21 1.39 14.2254 3 0 1 7.54 1.16 11.7236 4 0 1 7.79 1.55 11.3505 0 0 2 7.95 2.49 11.1143 1 0 2 11.23 1.02 7.8815 5 0 2 12.44 1.48 7.1127 6 0 2 15.21 1.72 5.8253 6 0 3 16.19 2.96 5.4762 10 0 0 16.65 1.03 5.3235 10 0 1 16.91 2.09 5.2426 4 0 4 17.60 2.74 5.0389 5 0 4 18.41 6.27 4.8195 6 0 4 18.71 2.45 4.7418 9 0 3 19.32 3.50 4.5938 7 0 4 19.45 5.51 4.5635 12 0 0 19.62 1.24 4.5247 1 0 5 19.84 1.50 4.4740 12 0 1 20.33 1.56 4.3690 8 0 4 20.61 1.37 4.3094 4 0 5 20.98 1.21 4.2341 12 0 2 21.18 4.16 4.1941 5 0 5 21.36 2.94 4.1590 11 0 3 21.41 2.21 4.1502 9 0 4 21.45 5.84 4.1418 13 0 1 21.86 13.61 4.0650 6 0 5 22.30 6.86 3.9868 2 1 0 22.47 1.63 3.9573 1 1 1 22.56 4.47 3.9407 10 0 4 22.64 35.60 3.9268 7 0 5 22.64 2.42 3.9267 2 1 1 22.73 5.20 3.9116 14 0 0 22.76 1.47 3.9079 12 0 3 23.07 4.23 3.8548 14 0 1 23.34 1.02 3.8109 4 1 1 23.48 2.15 3.7884 1 1 2 23.51 1.55 3.7836 8 0 5 23.51 2.57 3.7835 0 0 6 23.65 3.26 3.7615 2 1 2 23.93 7.88 3.7179 3 1 2 24.06 5.12 3.6981 14 0 2 24.14 4.82 3.6864 6 1 0 24.18 1.95 3.6806 13 0 3 24.32 5.57 3.6593 4 1 2 24.41 4.30 3.6468 4 0 6 24.46 60.51 3.6388 9 0 5 24.46 8.97 3.6387 6 1 1 24.70 11.76 3.6045 15 0 1 24.82 3.75 3.5879 5 1 2 24.90 49.05 3.5761 5 0 6 25.04 4.69 3.5568 0 1 3 25.04 1.09 3.5564 12 0 4 25.09 12.09 3.5493 1 1 3 25.17 6.99 3.5386 7 1 1 25.25 4.35 3.5272 2 1 3 25.40 12.91 3.5061 6 1 2 25.48 64.32 3.4952 10 0 5 25.49 10.40 3.4951 6 0 6 25.51 9.80 3.4911 3 1 3 25.63 8.14 3.4754 15 0 2 25.88 8.57 3.4425 4 1 3 25.96 41.63 3.4327 8 1 1 26.03 100.00 3.4226 16 0 0 26.08 18.08 3.4163 7 1 2 26.16 18.13 3.4061 7 0 6 26.33 1.53 3.3844 16 0 1 26.35 36.95 3.3829 5 1 3 26.35 1.20 3.3825 13 0 4 26.57 3.08 3.3546 11 0 5 26.85 1.88 3.3207 8 1 2 26.90 27.16 3.3140 6 1 3 26.93 6.78 3.3114 8 0 6 27.19 19.22 3.2798 1 1 4 27.21 7.05 3.2769 16 0 2 27.34 1.27 3.2623 2 1 4 27.48 36.93 3.2457 10 1 0 27.55 7.73 3.2379 7 1 3 27.58 4.40 3.2337 3 1 4 27.69 9.15 3.2215 9 1 2 27.70 4.28 3.2207 14 0 4 27.70 1.23 3.2205 2 0 7 27.77 28.19 3.2130 9 0 6 27.77 1.51 3.2130 10 1 1 27.93 11.07 3.1950 4 1 4 27.94 5.53 3.1930 3 0 7 28.28 18.44 3.1562 8 1 3 28.36 1.64 3.1471 5 1 4 28.61 4.48 3.1206 10 1 2 28.68 2.26 3.1128 10 0 6 28.71 3.84 3.1095 5 0 7 28.88 5.26 3.0915 6 1 4 28.91 2.75 3.0880 13 0 5 29.08 3.54 3.0707 9 1 3 29.59 1.11 3.0193 11 1 2 29.63 11.62 3.0154 18 0 1 29.64 26.78 3.0138 0 1 5 29.66 14.64 3.0123 11 0 6 29.69 16.44 3.0093 1 1 5 29.82 3.55 2.9958 7 0 7 29.82 15.56 2.9957 2 1 5 29.84 1.70 2.9942 12 1 1 29.96 29.54 2.9829 10 1 3 30.05 3.61 2.9736 3 1 5 30.15 2.53 2.9639 17 0 3 30.17 7.72 2.9621 8 1 4 30.63 3.97 2.9190 12 1 2 30.70 4.76 2.9127 12 0 6 30.77 2.21 2.9058 5 1 5 30.90 66.50 2.8941 11 1 3 30.93 12.42 2.8911 9 1 4 30.96 1.49 2.8882 13 1 1 31.72 1.72 2.8206 13 1 2 31.82 4.54 2.8123 7 1 5 31.88 6.50 2.8067 14 1 0 32.04 1.12 2.7936 19 0 2 32.13 1.17 2.7855 14 1 1 32.22 1.85 2.7786 4 0 8 32.50 15.02 2.7548 1 1 6 32.63 40.76 2.7444 2 1 6 32.65 23.61 2.7423 11 1 4 32.71 13.30 2.7381 20 0 0 32.84 2.45 2.7273 3 1 6 32.87 4.81 2.7247 14 1 2 32.95 1.29 2.7184 20 0 1 32.96 17.22 2.7176 13 1 3 32.96 3.76 2.7173 11 0 7 33.13 2.62 2.7039 4 1 6 33.50 1.30 2.6748 5 1 6 33.61 14.24 2.6665 12 1 4 33.67 1.32 2.6617 20 0 2 33.95 4.85 2.6404 10 1 5 34.07 1.49 2.6318 15 1 2 34.07 1.33 2.6315 14 1 3 34.48 6.26 2.6013 7 1 6 34.92 3.59 2.5697 13 0 7 35.23 3.00 2.5476 15 1 3 35.36 1.96 2.5382 16 0 6 35.53 9.92 2.5265 0 1 7 35.63 1.15 2.5197 1 0 9 35.70 2.25 2.5149 12 1 5 35.88 1.38 2.5026 3 1 7 35.91 1.09 2.5008 17 1 1 36.08 2.12 2.4892 22 0 0 36.15 2.62 2.4845 4 1 7 36.43 3.41 2.4662 16 1 3 36.58 1.85 2.4565 17 1 2 36.64 1.42 2.4527 17 0 6 36.80 16.11 2.4422 15 1 4 37.02 1.43 2.4281 18 1 0 37.67 5.20 2.3876 17 1 3 37.68 1.76 2.3874 14 1 5 37.96 2.49 2.3703 16 1 4 37.96 1.32 2.3702 8 1 7 38.00 1.74 2.3680 23 0 1 38.64 1.16 2.3302 23 0 2 38.74 2.10 2.3242 15 1 5 38.85 2.04 2.3180 1 1 8 38.96 1.49 2.3119 18 1 3 38.96 17.16 2.3118 2 1 8 39.03 1.63 2.3076 13 1 6 39.14 2.97 2.3016 3 1 8 39.16 25.98 2.3002 17 1 4 39.24 2.29 2.2958 19 1 2 39.43 1.14 2.2854 17 0 7 39.68 1.82 2.2712 23 0 3 39.71 3.22 2.2697 5 1 8 39.74 6.96 2.2682 1 0 10 39.85 3.18 2.2623 2 0 10 39.85 7.58 2.2619 16 1 5 40.00 1.41 2.2542 14 1 6 40.02 4.29 2.2527 3 0 10 40.27 1.31 2.2395 4 0 10 40.27 2.36 2.2393 19 1 3 40.41 2.80 2.2323 18 1 4 40.62 11.31 2.2210 20 1 2 40.66 1.27 2.2188 18 0 7 40.67 1.68 2.2182 20 0 6 41.01 5.82 2.2008 17 1 5 41.44 7.99 2.1792 21 1 1 41.63 4.34 2.1697 20 1 3 41.94 6.43 2.1543 19 0 7 42.03 2.97 2.1497 21 1 2 42.08 17.40 2.1471 21 0 6 42.27 6.71 2.1380 0 1 9 42.30 19.76 2.1364 1 1 9 42.65 1.86 2.1198 14 0 9 42.81 3.33 2.1124 4 1 9 42.89 1.27 2.1086 23 0 5 42.89 3.70 2.1086 22 1 1 42.94 1.56 2.1062 26 0 0 43.01 2.31 2.1030 21 1 3 43.11 1.74 2.0984 5 1 9 43.18 1.18 2.0951 17 1 6 43.24 1.58 2.0921 20 0 7 43.44 5.09 2.0830 19 1 5 43.47 1.51 2.0818 22 1 2 43.47 5.26 2.0817 6 1 9 43.52 4.39 2.0795 22 0 6 43.62 1.30 2.0752 15 0 9 43.90 7.62 2.0623 1 0 11 44.33 4.90 2.0435 25 0 4 44.33 19.50 2.0435 18 1 6 44.39 6.43 2.0407 4 0 11 44.42 14.82 2.0394 22 1 3 44.58 17.65 2.0327 16 1 7 44.59 1.38 2.0322 21 0 7 44.68 4.90 2.0280 5 0 11 44.72 5.02 2.0266 20 1 5 44.94 9.70 2.0172 23 1 2 44.99 5.98 2.0151 23 0 6 44.99 63.50 2.0148 0 2 0 45.04 1.98 2.0129 6 0 11 45.45 1.21 1.9955 14 1 8 45.52 8.73 1.9928 19 1 6 45.55 7.46 1.9916 10 1 9 45.73 1.67 1.9841 22 1 4 45.88 3.84 1.9780 24 1 1 45.91 3.96 1.9766 1 1 10 45.96 2.48 1.9746 22 0 7 46.00 8.92 1.9729 25 0 5 46.24 2.37 1.9634 14 0 10 46.37 3.41 1.9581 15 1 8 46.39 14.39 1.9575 4 1 10 46.43 2.34 1.9558 28 0 0 46.46 1.36 1.9544 9 0 11 46.74 2.63 1.9436 18 1 7 46.75 3.47 1.9432 20 1 6 47.06 1.10 1.9311 10 0 11 47.33 1.47 1.9205 24 1 3 47.33 2.93 1.9205 16 1 8 47.37 2.27 1.9192 23 0 7 47.37 3.37 1.9192 22 1 5 47.41 2.73 1.9175 7 1 10 47.59 10.87 1.9106 26 0 5 47.70 1.35 1.9065 13 1 9 47.70 2.45 1.9064 11 0 11 47.94 2.75 1.8974 25 1 2 47.99 1.09 1.8957 25 0 6 48.22 1.61 1.8873 2 0 12 48.37 1.22 1.8817 3 0 12 48.39 3.67 1.8810 9 1 10 48.41 2.19 1.8804 12 0 11 48.58 2.74 1.8739 4 0 12 48.79 8.84 1.8666 26 1 0 49.10 4.87 1.8556 17 0 10 49.19 1.17 1.8524 6 0 12 49.40 9.15 1.8449 15 1 9 49.40 5.24 1.8449 18 1 8 49.49 1.19 1.8419 26 1 2 50.05 3.69 1.8226 25 1 4 50.28 11.49 1.8147 12 1 10 50.32 1.74 1.8133 16 1 9 50.37 1.22 1.8116 5 1 11 50.52 1.15 1.8065 9 0 12 50.54 2.20 1.8059 27 1 1 50.56 2.58 1.8053 6 2 5 50.70 1.25 1.8007 6 1 11 50.94 7.07 1.7926 7 2 5 51.08 3.57 1.7882 7 1 11 51.08 4.34 1.7881 10 0 12 51.23 5.06 1.7834 19 0 10 51.29 2.15 1.7814 17 1 9 51.58 1.38 1.7719 25 1 5 51.66 1.08 1.7693 14 2 2 51.69 1.08 1.7684 11 0 12 51.72 2.23 1.7673 16 0 11 51.80 1.37 1.7650 14 1 10 51.87 13.18 1.7627 9 2 5 51.90 9.45 1.7619 27 1 3 51.92 2.79 1.7612 31 0 1 52.00 2.46 1.7587 15 2 1 52.00 6.88 1.7585 9 1 11 52.10 10.88 1.7554 5 2 6 52.14 1.52 1.7542 28 1 1 52.30 1.13 1.7493 18 1 9 52.42 15.18 1.7456 10 2 5 52.42 2.75 1.7456 6 2 6 52.42 1.60 1.7455 31 0 2 52.48 1.08 1.7436 29 0 5 52.50 2.00 1.7431 15 2 2 52.51 1.21 1.7427 2 0 13 52.67 3.63 1.7377 17 0 11 52.72 25.00 1.7363 16 2 0 52.79 4.74 1.7342 7 2 6 53.04 1.83 1.7264 26 1 5 53.06 1.03 1.7259 27 1 4 53.07 1.25 1.7257 13 0 12 53.11 1.45 1.7245 5 0 13 53.15 2.40 1.7233 11 1 11 53.22 1.88 1.7213 8 2 6 53.38 1.88 1.7164 16 2 2 53.51 1.95 1.7125 16 1 10 53.55 7.17 1.7113 32 0 0 53.66 1.16 1.7081 14 2 4 53.70 7.72 1.7070 9 2 6 53.80 1.58 1.7040 3 2 7 53.83 1.36 1.7030 14 0 12 54.03 1.13 1.6972 22 1 8 54.20 1.35 1.6922 32 0 2 54.25 1.16 1.6909 5 2 7 54.61 4.12 1.6806 28 1 4 54.70 1.57 1.6779 19 0 11 54.80 3.16 1.6753 18 2 1 54.82 4.07 1.6747 11 2 6 54.92 1.08 1.6719 7 2 7 55.21 1.13 1.6637 10 0 13 55.45 1.22 1.6570 12 2 6 55.52 1.01 1.6550 33 0 1 55.78 1.84 1.6480 20 0 11 56.72 4.24 1.6228 20 2 0 56.89 1.29 1.6185 11 2 7 57.35 2.93 1.6065 18 0 12 57.48 1.76 1.6033 20 1 10 57.82 1.02 1.5946 14 0 13 58.18 1.34 1.5856 13 2 7 58.60 1.31 1.5753 15 0 13 59.99 1.44 1.5421 32 1 3 60.90 1.12 1.5212 36 0 0 61.20 1.52 1.5145 18 0 13 61.32 1.18 1.5118 12 1 13 61.57 3.03 1.5063 1 2 10 61.64 1.31 1.5046 2 2 10 61.76 1.10 1.5021 25 0 11 61.77 1.91 1.5018 3 2 10 61.83 3.27 1.5004 26 1 9 62.72 1.41 1.4813 33 1 4 63.19 3.00 1.4716 19 2 7 63.19 2.58 1.4715 27 1 9 63.30 8.52 1.4693 21 2 6 63.36 1.04 1.4680 25 1 10 64.02 1.06 1.4544 23 1 11 64.21 1.23 1.4506 10 1 14 64.38 2.37 1.4470 22 2 6 64.68 4.08 1.4412 1 2 11 65.00 2.46 1.4348 25 2 4 65.05 3.41 1.4338 4 2 11 65.28 2.39 1.4294 5 2 11 65.51 3.13 1.4248 23 2 6 65.55 1.06 1.4240 6 2 11 65.93 2.03 1.4169 32 1 7 66.18 2.52 1.4121 36 1 2 66.27 1.40 1.4103 22 2 7 66.31 4.88 1.4096 25 2 5 66.33 1.25 1.4093 4 1 15 66.42 1.22 1.4075 25 1 11 66.43 2.10 1.4074 26 0 12 66.49 1.20 1.4062 14 2 10 66.55 5.78 1.4051 5 1 15 66.64 1.31 1.4034 28 2 0 66.82 3.73 1.4000 6 1 15 66.90 1.86 1.3986 36 1 3 67.32 2.71 1.3909 34 1 6 67.33 1.60 1.3907 28 1 10 67.39 1.28 1.3897 23 2 7 67.44 2.47 1.3886 35 1 5 67.55 1.89 1.3867 37 1 1 67.57 6.34 1.3864 26 2 5 67.66 1.34 1.3848 11 2 11 67.98 1.43 1.3790 37 1 2 68.02 2.06 1.3783 37 0 6 68.07 1.03 1.3774 2 2 12 68.08 2.29 1.3772 32 1 8 68.22 1.31 1.3747 12 2 11 68.28 1.24 1.3736 38 0 5 68.36 1.58 1.3722 4 2 12 68.40 1.77 1.3716 10 1 15 68.78 2.83 1.3649 17 2 10 68.91 1.08 1.3627 11 1 15 68.96 4.70 1.3618 31 1 9 68.98 1.31 1.3615 27 1 11 69.01 1.13 1.3609 35 1 6 69.18 1.92 1.3580 36 1 5 69.68 1.13 1.3495 37 1 4 70.37 1.13 1.3379 1 1 16 70.40 2.69 1.3374 10 2 12 70.48 1.78 1.3360 32 1 9 70.52 3.17 1.3354 19 2 10 70.93 1.14 1.3287 26 1 12 70.93 1.42 1.3286 16 2 11 71.10 1.81 1.3260 31 2 1 71.51 1.05 1.3193 31 2 2 71.73 2.46 1.3159 17 2 11 71.73 1.67 1.3159 8 3 1 71.91 1.48 1.3130 5 3 3 72.16 1.12 1.3091 16 1 15 72.17 1.26 1.3089 6 3 3 72.45 1.68 1.3046 10 3 0 72.46 4.84 1.3043 32 2 0 72.74 1.14 1.3000 10 1 16 72.95 1.41 1.2969 17 1 15 73.13 2.97 1.2941 32 1 10 73.19 1.64 1.2932 33 0 11 73.19 1.46 1.2932 25 1 13 73.44 1.08 1.2894 19 2 11 73.51 1.16 1.2883 35 0 10 73.53 1.28 1.2880 0 3 5 73.69 1.45 1.2856 10 3 3 74.19 3.49 1.2782 11 3 3 74.36 1.32 1.2757 20 2 11 74.66 2.25 1.2713 33 1 10 75.13 2.31 1.2645 2 3 6 75.15 1.35 1.2643 11 3 4 75.56 1.45 1.2583 20 1 15 75.72 2.00 1.2561 18 2 12 75.93 1.86 1.2532 42 0 5 75.95 1.23 1.2529 5 0 18 76.52 5.04 1.2449 21 1 15 76.84 2.04 1.2405 42 1 0 77.08 1.40 1.2373 41 1 4 77.53 3.50 1.2313 22 1 15 77.58 1.21 1.2306 15 3 4 78.93 1.40 1.2129 2 3 8 79.06 2.29 1.2112 17 3 4 79.11 1.24 1.2106 18 2 13 79.48 1.03 1.2058 14 1 17 80.01 1.01 1.1992 20 3 2 80.21 1.79 1.1967 42 1 5 81.14 2.11 1.1853 1 3 9 82.54 2.14 1.1687 18 3 6 82.61 1.66 1.1679 22 3 3 82.72 2.05 1.1667 16 3 7 82.96 1.02 1.1639 44 1 4 82.97 1.05 1.1637 23 3 2 83.27 1.62 1.1603 19 1 17 83.85 1.84 1.1538 26 2 12 84.02 1.81 1.1519 4 3 10 85.32 1.90 1.1376 37 2 6 85.57 1.12 1.1349 38 2 5 85.80 1.18 1.1325 26 3 0 86.08 1.01 1.1296 26 1 16 86.26 1.36 1.1276 15 3 9 86.93 1.55 1.1206 12 3 10 88.19 1.35 1.1078 27 3 3 ================================================================================ XPOW Copyright 1993 Bob Downs, Ranjini Swaminathan and Kurt Bartelmehs For reference, see Downs et al. (1993) American Mineralogist 78, 1104-1107.