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Volume 2, pages 63-64, 1917 THE ORIGIN OF THE MIRABILITE FROM THE ISLE ROYALE MINE. ALFRED C. LANE Tufts College The water of this mine, like that of all Michigan copper mines runs very high in calcium chloride. The nearest samples analyzed to the place where the mirabilite was found give the following results, in parts per thousand .2 1 2 Cl........................ 45.178 7.626 Ca........................ 22.201 3.249 Na........................ 3.837 1.204 Br......................... 0.320 SO4..................... 0.234 Cu.......................... a Sum .................. 71.770 12.079 Difference............ 0.330 0.241 Total solids.......... 72.100 12.320 Sp.gr.................... 1.057 1.009 a Estimated as 2 to 3 mg. per liter. The sample is, however, somewhat nearer a fault which crosses the lode obliquely striking E and W and heading 30° or less to the N and it is conceivable that a certain amount of water may circulate along this fault. These faults or crossings also contain more sulfides so that the water near them may well contain more sulfates. The interesting thing about this occurrence of mirabilite is that while it seems to have deposited from water so high in lime, it is yet so free from lime. To explain this we may note that calcium sulfate is much less soluble than sodium sulfate, and still less so in the presence of calcium chloride. For instance at 0° C. 0.241 gram of gypsum is dissolved by 100 grams of water, and at 18°, 0.259 gram; whereas in a solution containing 15.90% calcium chloride at 25°, only 0.092 gram of gypsum is dissolved in 100 cubic centimeters. 3 Calcium sulfate is more soluble in sodium chloride solutions, because sodium sulfate is formed, and this is enormously more soluble.Unfortunately I did not observe the temperature of the mine and rock at the time we took the sample, but the normal temperature cannot be far from 15° C. (61° F.). At that temperature we find the following peculiar behavior of sodium sulfate: 37.43 grams of sodium sulfate or 105.79 grams of sodium sulfate +7 aq. can exist away from the air, but by exposure to air (or the inoculation with crystals) it is converted to the sodium sulfate with 10 aq., of which only 35.96 grams (corresponding to 13.20 grams of anhydrous sodium sulfate) are soluble. This solubility decreases markedly with the drop in temperature, so that at 10° but 9 grams and at 0° but 5.02 are dissolved. Therefore both exposure to air and cooling would tend to promote crystallization of mirabilite more than gypsum. A study of the occurrence of this efflorescence with reference to the season and its relation to upcast and downcast currents in the mine circulation would show if temperature were the more important factor. The rock of the upper levels is down to 6° C. (43° F.) and the air temperature of the downcast shaft generally below 15° C. (60° F.). NOTES 2 Mich. Geol. Biol. Survey, Publ. 6, Geol. Ser. 4, 11, 823. 3 Chemiker Kalender, Pt. 1, 323 and 332, 1916.
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