Norbergite |
| Mg(OH,F)2.Mg2SiO4 |
| Orthorhombic |
| a : b : c = 1.10 : 1 : 1.897 ; po = 0.723, qo = 1.897 |
Forms
c(001), D(110), M(011), N(021), e(111), d(221)
Crystallography
Crystals of norbergite are very rare, the only measurable one known having
been presented to the Harvard Mineralogical Museum in 1906 by Dr. Ungemach,
of Paris, who found it with a few others in the Nicoll limestone quarry at
Franklin, then active. This crystal, about half an inch in diameter, is shown
in figure 150 and from a different position in Berman's paper (261).
| Figure 150 Crystal of norbergite, thick-tabular parallel to the base, drawn with the brachyaxis in front and showing the forms c(001), M(011), N(021), D(110), e(111), and d(221). Nicoll quarry, Franklin. |
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The relation of norbergite to the other members of the group is discussed at length there and need not be considered here. It is enough to say that the angles are practically the same as those of humite and the crystal was supposed to be humite until analyzed.
Physical properties
Norbergite ranges in color from tawny (Ridgway's 13’’j) to chamois
(Ridgway's 19’’b), and the lighter-colored varieties have lower
indices of refraction than the darker ones, indicating a lower iron content.
It has a hardness of 5.5 and in general appearance resembles the other members
of the humite group. Its specific gravity is 3.20.
The optical properties of norbergite from Franklin are similar to those reported by Geijer for the mineral from Norberg. It is positive; 2V is 50° about; the dispersion is slight, and it is slightly pleochroic. The refractive indices differ somewhat—those of the darker, analyzed material are: a = 1.565, b = 1.570, g = 1.591, 2V = 49° ; those of the chamois-colored crystals are: a = 1.561, b = 1.566, g = 1.587, 2V= 50° , and those found by Geijer (245a) for the Norberg mineral are: a = 1.563, b =1.567, g =1.590, 2V = 49° 30'.
Composition
Norbergite is a basic magnesium fluosilicate in which the molecular ratio
of Mg(OH,F)2 to Mg2SiO4 is 1 to 3. The analyzed
material from Franklin falls short of satisfying the accepted formula, as
it contains far too little fluorine, and the hydroxyl is almost negligible.
|
1 |
2 |
3 |
|
| SiO2 |
28.63 |
28.54 |
29.60 |
| Al2O3 |
3.12 |
0.09 |
|
| Fe2O3 |
0.69 |
||
| FeO |
0.73 |
0.82 |
|
| MgO |
59.42 |
56.45 |
59.56 |
| CaO |
1.01 |
||
| MnO |
0.04 |
0.18 |
|
| F |
12.78 |
13.70 |
18.72 |
| H2O |
0.32 |
2.90 |
|
| CO2 |
1.21 |
||
|
105.04 |
105.59 |
107.88 |
|
| O = F2 |
5.38 |
5.77 |
7.88 |
|
99.66 |
99.82 |
100.00 |
| 1. Norbergite crystal, containing about 2 percent of chondrodite, Franklin. L. H. Bauer (261), analyst. |
| 2. Norbergite, Norberg, Sweden. Per Geijer (245a), analyst. |
| 3. Composition of theoretical mineral: MgF2.Mg2SiO4. |
Occurrence
The tawny-yellow crystal that was measured, and others of paler color and
not so sharply developed, were found in the Nicoll quarry of the Franklin
Iron Company. In 1905 the author found norbergite abundant in the Fowler quarry,
in granular masses associated with black spinel, fluorite, and pyrrhotite.
Plate 17, B, shows the abundance of the
mineral. He also found very pale yellow grains, but at that time no crystals,
in the Iron Company's quarry.
The Parker shaft encountered several pegmatite dikes cutting the Franklin limestone. According to Nason (163) the limestone adjacent to the dikes contained much chondrodite, together with phlogopite and tremolite. The pegmatitic origin of norbergite and chondrodite could not be established in the other localities mentioned, as it was at the Parker shaft, but it is considered probable at all the localities where those minerals were found.
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Website
© by Herb Yeates 1997-2006.
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page created: August 12, 2006 6:42 PM
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