MINERALS INDEX
Spinel |
| (Mg,Fe)O.(Al,Fe)2O3 |
| Isometric |
Forms
o(111), d(110), and a(100)
Habit
Spinel forms octahedral crystals, some of them showing faces of the dodecahedron
and more rarely of the cube. Twinning, when it occurs, is on the spinel law.
The color of the crystals is dark gray, green, or black.
Composition
The following analysis shows the-composition of spinel:
|
1 |
2 |
3 |
||
| Al2O3 |
66.09 |
67.17 |
0.659* |
0.671 |
| Fe2O3 |
1.91 |
0.012* |
|
|
| FeO |
10.64 |
8.91 |
0.124† |
0.670 |
| MgO |
21.66 |
22.01 |
0.546† |
|
| SiO2 |
0.80 |
|||
| H2O |
0.96 |
|||
|
100.15 |
100.00 |
|||
| [* Figures reflected in the value 0.671 shown] |
| [† Figures reflected in the value 0.670 shown] |
| 1. Ceylonite. Vogel (79), analyst. |
| 2. Same analysis, recomputed to 100 percent after omitting SiO2 and H2O and distributing the iron between the ferrous and ferric oxides so as to give the spinel ratio. |
| 3. Molecular ratio calculated from the analysis. |
The analysis, made by Vogel in Rammelsberg's laboratory, is of crystals described as green-black octahedrons, associated with zincite and calcite, an association that suggests gahnite. The analysis, however, shows no zinc but, until recomputed as in column 2, gives poor ratios for the spinel formula. An earlier analysis of Franklin spinel by Thomson (23) must be rejected as giving a composition wholly abnormal for the species.
Occurrence
Most of the "spinel" found near Franklin is gahnite and is so described
in this paper. Some true spinel has been found, but the distinction can be
made only by chemical tests. A few octahedrons of green and brown spinel are
found in the pegmatite associated with the ore body of the Buckwheat mine;
one in the Hancock collection was originally coated with cleiophane.
In the Franklin limestone many so-called "pockets" have been found, in which spinel is associated with corundum, rutile, phlogopite, and hornblende. One of these pockets, on Wildcat Branch, south of the Gooseberry iron mine, yielded sharp octahedrons of purplish spinel as much as an inch across, in limestone with serpentine. In the limestone quarries at Franklin small black crystals of spinel associated with norbergite were not uncommon in 1905, and at the Rudeville quarries brown crystals, with chondrodite, were at one time abundant.
The Canfield collection is especially rich in specimens of gray, green, and black spinel from several localities in the neighboring towns of Sparta and Newton, all in limestone near its contact with granite gneiss. The most noteworthy specimen is a large cluster of gray octahedrons from Sparta, the largest 17 inches around the base. Another large, isolated twin crystal of perfect symmetry came from the same town, and with it was found clear, red spinel, the only occurrence of the sort known in the region.
The large black crystals of iron spinel found in the towns of Amity and Warwick, N.Y., occur in the northward extension of the Franklin white limestone.
In the collection of Mr. Rowe at Franklin was seen a single specimen of yellow spinel embedded in limestone, which presents the form of a simple cube. The cube has been deformed by pressure but is capable of contact measurement and shows scarcely a trace of the octahedron. It is unique among Franklin specimens of spinel in its cubic form and is an interesting link with the rare cubic forms of gahnite and franklinite also found there.
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Website
© by Herb Yeates 1997-2006.
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page created: August 12, 2006 6:55 PM
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