Jeffersonite |
Habit
Jeffersonite is found in large crystals of simple habit, some more than 12
inches long and many doubly terminated. It occurs also in granular and platy
masses. Contact twins on the orthopinacoid are not rare, and some lamellar
twins on that plane give a good parting.
| Figure 77 Crystal of jeffersonite showing the b(010), a(100), m(110) and s(111). Sterling Hill. |
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Figure
78 Crystal of jeffersonite twinned on the orthopinacoid, showing the forms of figure 77. Sterling Hill. |
Lamellar twinning on the base with perfect parting is the rule. There is a poor cleavage parallel to the prism and in traces parallel to the clinopinacoid. The hardness is 5.5, and the specific gravity is 3.55 to 3.63. The color is dark olive-green to brown and on weathered surfaces is gray to chocolate-brown and black. The mineral is translucent and in thin section is yellowish.
Optical properties
The jeffersonite of analysis no. 7 is
optically biaxial and positive; 2V = 74° (on Fedorov stage);
Z /\ c = 55° ;
r < v (easily perceptible); an optic axis emerges from the basal parting.
a = 1.713, b
= 1.722, g = 1.745, all ±0.003. Pleochroism:
X and Y = olive-green, Z = brownish-green (Berman).
Composition
Analyses 6 and 7 (see page 62) show the
composition of a pyroxene containing manganese, zinc, and iron in different
proportions in addition to calcium and a little magnesium. Jeffersonite is
distinguished from schefferite by the presence of zinc and of more than traces
of iron.
Occurrence
Jeffersonite is abundant, both in the pegmatite and in the contact deposits
in the limestone. At Franklin it was found chiefly in granular form as the
principal dark mineral of the pegmatite dikes at the Trotter mine and at the
Parker shaft. It was also abundant in limestone in those mines, near pegmatite
contacts, either in coarse granular masses or in rude crystals isolated in
the limestone, commonly with garnet, rhodonite, microcline, franklinite, and
gahnite. The striking characteristics of jeffersonite are its dark-green color,
its vitreous to greasy luster, and its brilliant basal parting.
A rare mode of occurrence of jeffersonite was observed in the veins containing hetaerolite and hodgkinsonite, described on page 49, where it forms a matrix for those minerals and where its dark color caused it to be mistaken at first for a manganese oxide such as hetaerolite.
At Sterling Hill jeffersonite is one of the most typical minerals of the contact deposits about the pegmatite. Thence came the abundant crystals, many of them large and doubly terminated, that early drew attention to the species. Plate 8, A, shows one of unusual size.
| Figure
79 Crystal of jeffersonite showing the forms b(010), a(100), m(110), z(021), p(101), s(111) and o(221). Sterling Hill. |
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Most of the crystals are deeply pitted or have rounded edges and a dull coating of altered material. They were found in limestone pockets or in the residual clay of the limestone, with dysluite (gahnite), garnet, and apatite. The pegmatite also contains jeffersonite along with black manganiferous hornblende, and much of the jeffersonite contains grains of galena and sphalerite.
Alteration
Crystals of dark-brown jeffersonite, perfectly preserved in form but as light
as pith and showing a porous texture when broken, were once very abundant
at Sterling Hill. To them Koenig (117) gave the provisional name "anomalite"
but without publishing any exact description of the analyzed material. He
regarded the material as being the last stage of the alteration of jeffersonite
and stated that it consists of iron and manganese hydroxides with small amounts
of cobalt and nickel. The name referred to its supposed anomalous behavior
when dissolved in a borax bead—the red color that should have been given
by the manganese oxide being neutralized by the nickel and cobalt oxides,
so that the bead was colorless. No further analytical study of these pseudomorphs
has been made.
In the Hancock collection is a specimen of jeffersonite of ordinary crystal form and dull luster showing patches of jet-black hornblende with brilliant facets, embedded in but not projecting above the pyroxene surface. The mineral appears to be the result of alteration to amphibole, but it has not been more closely studied.
Historical notes
Jeffersonite was first described from Sterling Hill by Vanuxem and Keating
(8), who named it for President Jefferson. They gave analyses and recognized
it as a pyroxene, but they had no crystals and could not prove its relation
to augite. That was done in the following year by Troost (13), who gave figures
of the crystals and showed their pyroxene nature. He described the abundance
of the mineral at Sterling Hill in the pegmatite and in the contact ore deposits.
The paper by Troost is of much interest to American mineralogists as being
among the first published in this country treating of crystallography and
giving crystal figures. Seybert (19) also gave an analysis and confirmed the
pyroxenic nature of jeffersonite. These early analyses are too poor to justify
republication here.
The variable but never-failing content of zinc in this pyroxene throughout its wide range of occurrence in the deposits marks it as a distinct variety. Also it is a very characteristic member of the contact zones of the district.
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© by Herb Yeates 1997-2006.
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