Occurrence
At Franklin greenish-yellow or blue octahedral crystals are not uncommon in the limestone wall rock and the ore body at the Trotter mine and southward along the vein. The name "automolite" was used by the earlier writers, beginning with Nuttall (7) and Vanuxem (9), for the greenish crystals, but no analysis of them is recorded. There is no reason to doubt the correctness of Alger's conclusion (41) that they were the same mineral as the dysluite from Sterling Hill. Abundant sapphire-blue octahedral crystals of gahnite, reaching a diameter of three-quarters of an inch, were collected by J. E. Wolff in the wall rock of the Trotter mine in 1896. They are associated with gray pyroxene, titanite, and lollingite, embedded in coarse white limestone.

Gahnite of very unusual cubic habit was found by Brush in 1869 in a tunnel driven from the Wallkill River through the wall rock of the Buckwheat mine. The gahnite was in the Franklin limestone at its contact with either the ore body or the gneiss; the exact mode of occurrence was not observed, and the locality could never be relocated afterward. These crystals, the better part of which are now in the Brush collection at New Haven, range in size from an eighth of an inch to 1-½ inches, most of them-small, and are of blackish-green color. Their hardness is 7.5, and their specific gravity is 4.9. Associated with them in the limestone were biotite, apatite, roepperite, and lollingite. They are unique among spinels (but see the description of franklinite, page 45) in their dominant cubic habit, the faces of the cube being as smooth and brilliant as those of the other forms. The cube is modified by the octahedron and dodecahedron and by minute planes of q, n, (411), and (811), the last two rounded and doubtful. The facts as to this occurrence of gahnite are reported by Brush, whose paper (100) included the analysis by Adam (no. 3, page 43) and was later illustrated with figures of the most typical crystals by Penfield (100, reprint).

At Sterling Hill beautiful crystals of gahnite, with jeffersonite and small crystals of brown garnet, were found very early by mineral collectors, in a large pocket or series of pockets in the limestone on the west wall of the eastern leg of the ore body. The best specimens are preserved in the Canfield collection and are among its greatest ornaments. The largest single crystal is a perfect octahedron 5 inches on an edge, of dark greenish-black color, with smooth bright faces. It is illustrated in plate 5, B. It is the crystal, collected by Mr. F. Canfield, to which Jackson refers in his paper (52).

Among many other specimens from the same locality in this collection are groups and separate crystals of gahnite in octahedrons from 4 inches on an edge downward in size, some attached to large crystals of jeffersonite) and either still embedded in or quite freed from the limestone matrix. Of the many crystals only one shows all the faces of the dodecahedron as narrow truncations; others show single sporadic faces of that form. Plate 5, A, illustrates some of these specimens.

Small crystals of similar green gahnite could in 1905 still be seen in the limestone wall of this part of the old Passaic mine, and at several other points spinel crystals of this type were seen, though not analyzed for zinc.

This mineral was first mentioned by Vanuxem and Keating (12) as new, under the name of dysluite, a name still used in the district to describe this occurrence. Dysluite was recognized by Dana as a varietal type, although as long ago as the time of Alger (41) the fact was recognized that there is no difference between it and the so-called "automolite" of Franklin. The crystal whose analysis is presented above (no. 4, page 43) was given for analysis by Mr. Canfield.