Prehnite is found in faintly yellowish-white coarse lamellar to platy aggregates, some of the plates being in fan-shaped groups. An imperfect pinacoidal cleavage shows a pearly luster. The specific gravity is 2.89 to 2.96. In appearance the mineral is wholly unlike any other prehnite known to the author, and its nature was not suspected until the results of the analyses were available.
Composition
Prehnite is an acid orthosilicate of calcium and aluminum. The early analyses gave quite unsatisfactory ratios for prehnite until it was discovered that the mineral was mixed with pectolite, but after correction for the presence of enough of that mineral to account for the alkalies found, both gave fairly satisfactory ratios. After it was learned that pectolite is fluorescent under the iron-arc spark, whereas prehnite is not, some of the material from which analysis 2 was made was again separated, first in a heavy solution and finally under the spark, and thus a sample almost free from pectolite was obtained. Analysis 3, made of this sample, shows the presence of only about 2 percent of impurity and gives an almost exact ratio for prehnite.
A specimen of more typical prehnite from Franklin, found in the Hancock collection, shows a perfect basal cleavage. It is optically biaxial and positive, 2V is medium to large, and Z is normal to the cleavage; a = 1.617, b = 1.625, g = 1.643, all ±0.002 ; b is parallel to the cleavage (Berman).
Occurrence
According to Mr. T. Lang, of Franklin, a considerable amount of the material of analysis 1 was found on the dump of the Parker shaft with the new silicates discovered in 1897. The analysis by Warren was made during the examination of the new material by Penfield and Warren, but as it gave a poor ratio it was never published, and the author is indebted to him for its use here. In the specimens seen by the author prehnite is intimately mixed with yellow garnet, grains of franklinite, and scales of biotite, all readily visible. After the analysis was made, grains of transparent white pectolite were discovered in the material, so intimately mixed with the prehnite that separation of either to a pure product is extremely difficult.
In the Hancock collection a single specimen of prehnite in more typical form was found. It was labeled simply "Franklin Furnace," and nothing is known of its source, but others resembling it were found on the dumps of the Parker shaft. In it rosettes and crusts of thin white plates of prehnite line cavities in a mass of manganese garnet and biotite. Its nature was established by optical determination by Mr. Berman, who furnished the optical data given above. |
