MINERALS INDEX
Sussexite |
| H(Mn,Mg,Zn)BO3 |
| Orthorhombic |
Physical character
Sussexite is found in silky fibers in parallel groups that constitute the
filling of thin veins, the fibers being either parallel to the vein walls
or transverse to them. It also forms felted fibrous masses.
Sussexite is white, with a tinge of yellow or pink, and translucent. Its luster is silky to pearly. Its hardness is 3, and its specific gravity is 3.42 (Brush) or 3.123 (Penfield).
Sussexite is biaxial and optically negative. Its fibers are parallel to the vertical axis. X = c’ , Z = bisectrix in the acute angle of the unit prism. a = 1.630, b = 1.712, g = 1.709 (Larsen).
Composition
Sussexite is an acid borate of manganese and magnesium, having the general
formula HRBO3, in which R includes manganese, magnesium, zinc,
and in some material iron and calcium, though the last two may be merely impurities.
|
1 |
2 |
3 |
4 |
5 |
|
| B2O3 |
31.89 |
33.31 |
33.16a |
29.20 |
30.52 |
| MnO |
40.10 |
38.08 |
37.58 |
47.27 |
49.40 |
| MgO |
17.03 |
15.92 |
16.29 |
9.15 |
9.56 |
| ZnO |
3.24 |
3.87 |
3.14 |
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| H2O - |
9.59 |
8.53 |
0.10 |
7.97 |
8.33 |
| H2O (250°) |
0.90 |
7.80 |
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| Fe2O3 |
0.60 |
||||
| FeO |
0.15 |
0.16 |
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| CaO |
0.10 |
1.94 |
2.03 |
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| SiO2 |
0.50 |
1.30 |
|||
|
98.61 |
99.98 |
100.00 |
100.12 |
100.00 |
| a Difference |
| 1. G. J. Brush (95), analyst. |
| 2. S. L. Penfield and E. S. Sperry (135), analysts. |
| 3. E. Pointevin and H. V. Ellsworth (229), analysts. |
| 4. L. H. Bauer (257), analyst. |
| 5. No. 4 recomputed to 100 percent after deducting 4.5 percent of willemite, equivalent to the SiO2 found. |
The molecular ratio of B2O3 : RO : H2O, computed from the analyses, is very nearly 1 : 2 : 1, from which the accepted formula is derived.
The analysis by Pointevin and Ellsworth (no. 3) was made in order to establish the optical characters of the mineral on analyzed material. The optical constants determined by them are essentially the same as those found by Larsen and given above.
Occurrence
Sussexite was first described by Brush (95) from material collected by Mixter
and himself at the Hamburg mine at Franklin, where it was associated with
carbonates of manganese and magnesium and blackened pyrochroite in secondary
veins cutting massive franklinite ore. Plate 19, B,
shows a typical but unusually large specimen. A later analysis of the original
material by Penfield (135) confirmed Brush's formula. Well-authenticated specimens
of silky fibrous sussexite, less compact than the type material and embedded
in massive ore, were seen in collections from the Parker shaft and from the
Taylor mine; it is reported also from the Trotter mine. Nevertheless it was
found in very small amounts at all localities, and but little of the mineral
is preserved in collections.
Much of what is labeled sussexite is a form of tremolite asbestos cemented by calcite or by the mixture of fibrous calcite arid zincite known locally as "calcozincite." From these sussexite can be readily distinguished by its characteristic flame reaction for boron as well as by its optical character.
During 1913 and 1914 considerable amounts of sussexite were found with pyrochroite and leucophoenicite (see page 51) in the northern part of the Parker mine. A specimen in the Harvard collection shows the delicate white fibers intergrown with pale-pink rhodochrosite in a thin vein.
In 1927 a specimen was found on the picking table at Franklin so different in appearance from normal sussexite that it was not recognized as such until analysis had proved its nature. The material forms a vein in massive ore, largely composed of yellow willemite intermingled with a dull-pink massive substance with the appearance of garnet, which is the sussexite. When crushed it shows under the microscope a felted fibrous texture quite unlike the usual parallel and separable fibrous texture of ordinary sussexite. It is biaxial and negative; 2V small, elongation negative, parallel extinction; a = 1.65, b = 1.71, g = 1.715. Analysis 4, made of this material, is that of a sussexite with much less magnesium and correspondingly more manganese than the type material. It constitutes a distinct facies of sussexite, which is one of the considerable number of minerals known only from Franklin.
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© by Herb Yeates 1997-2006.
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