Calcite |
| CaCO3 |
| Hexagonal-rhombohedral |
Forms
c(0001), a(1120), b(1010),
a(44-83), g(8.8.16.3),
*a.(1015),
b.(3.0.3.16),
b:.(0.3.3.16),
d.(0112),
p.(1011),
f.(0221),
.t:(2134),
A:(11.1.12.10), K:(2131),
Q:(19.13.32.6),
S:(11.18.19.3),
T:(4371), V:(6.5.11.1),
S:.(8.7.15.1), *W:.(18.4.22.5)
(Letters after Goldschmidt, Atlas, vol. 2.) The two forms marked with an asterisk (*) are new; their position angles are as follows. [Web Ed. note: forms here marked with :. were marked by three dots arranged vertically in the original.]
[Position angles of new forms of calcite; combinations on crystals of calcite]
Habit
Crystals of calcite are rare, and the
mineral is generally coarse to fine granular or compact, with cleavage rhombohedrons
as much as 6 inches thick, showing distinct striations due to pressure twinning.
A parting parallel to the twinning plane is nearly as perfect as the cleavage.
When freshly broken the mineral is gray or white or in manganiferous varieties
generally pink. On exposure to sunlight the manganiferous calcite becomes
at first yellowish brown and finally black.
Under the iron-arc spark the pure calcite shows no fluorescence, but the manganiferous varieties show various shades from vivid red to pale pink. Some material that is apparently calcite has a violet fluorescence, but the nature of the impurity is unknown.
In a recent paper Krieger (278a) has shown that the manganocalcite varies uniformly in specific gravity, refractive indices, and X-ray diffraction pattern with the chemical composition, increase of manganese causing increase of density and of both refractive indices.
Composition
Much of the white Franklin limestone
is essentially pure calcium carbonate. It shows, however, all gradations to
dolomite, the amount of magnesium present not markedly affecting the physical
appearance of the limestone. Many analyses have been published by Nason (141)
and by Kummel (190a), which are not here reproduced.
The gangue of the zinc ores is, however, commonly manganiferous, so that the waste dumps, when they have lain but a few months exposed to the weather, become superficially quite black through oxidation of the manganese. For such manganiferous calcite several names have been proposed—spartaite (Breithaupt, 73), manganocalcite and calcimangite (Shepard, 89), and roepperite (Kenngott, 103). The analyses listed below show that the composition is diverse, and only those specimens with relatively low manganese are shown here, others being classed as rhodochrosite. The cleavage angle of "spartaite", as recorded by Breithaupt, is 75° 21/2´. Material described by Levison (206) is in tough, hard, fibrous masses and in splinters as much as 6 inches long, with a specific gravity of 2.81.
Occurrence
Granular crystalline calcite is the
sole gangue mineral of both zinc-ore deposits, is the principal constituent
of the wall rocks and of the Franklin limestone as a whole, and is the chief
material filling the secondary transverse veins in the Franklin ore body.
It is in the veins that crystals are found, the few noteworthy occurrences
being described below.
In veins in the Buckwheat mine containing, among other species, zincite and pyrochroite (see page 39), calcite was found partly as poorly crystallized masses filling the vein, and partly as exquisite transparent crystals of slender prismatic habit, peculiar in the equal development of prisms of both orders. Combinations 14 and figure 57 illustrate this occurrence.
| Figure
57 Prismatic crystal of calcite showing the forms c(0001), a(1120), b(1010), p.(1011),) and d.(0112). Buckwheat mine. |
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Crystals of similar habit, with additional faces of the pyramids of the second order and rare scalenohedral forms, were found with the specimens of pyrochroite and hodgkinsonite from the Parker shaft. Some of them have the base replaced by flat positive rhombohedrons, of which the form a.(1015) may be regarded as certain, and a number of others of flatter slope are regarded as indicated. Combinations 511 and figures 58 and 59 illustrate these forms.
| Figure
58 Prismatic crystal of calcite showing the forms c(0001), a(1120), b(1010), p.(1011), a (44-83), and f.(0221). Parker shaft. |
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Figure
59 Crystal of calcite showing the c(0001), a(1120), p.(1011), d.(0112) and g(8.8.16.3). Parker shaft. |
Gordon (222) has figured a crystal from Franklin showing the new forms b.(3.0.3.16) and b:.(0.3.3.16) —very flat rhombohedrons. Crystals of calcite of the habit of combination 14 were abundant in the cavities containing larsenite.
A small specimen of manganite from Sterling Hill showed also brilliant crystals of calcite. They were in part simple scalenohedrons, as shown in figure 60, and in part more complex, with the new positive W:.(18.4.32.5), as shown in figure 61.
| Figure
60 Very acute scalenohedral crystal of calcite showing the forms p.(1011) and V:(6.5.11.1). Sterling Hill. |
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Figure
61 Scalenohedral crystal of calcite showing the forms p.(1011), a(1120), V :(6.5.11.1), Q:(19.13.32.6), and the new scalenohedron W:.(18.4.22.5). Sterling Hill. |
Scalenohedral crystals (combinations 1517) were found in the porous dolomite of the Buckwheat mine and in the Hamburg and Parker mines. Rhombohedral crystals (combinations 1820) were found in cavities in the Kittatinny limestone removed in building the foundations of the mill at Franklin.
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© by Herb Yeates 1997-2006.
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