MINERALS INDEX
Bementite |
| H10Mn8Si7O27 |
| Orthorhombic |
Physical character
Crystals of bementite are not known, but the mineral is found in massive granular form,
breaking into rectangular blocks; in radiate or stellate masses with minute foliate
structure; and in compact, hornlike form.
It has cleavages of different perfection in three mutually perpendicular directions and therefore parallel to the three pinacoids. Its hardness is 4.5, and its specific gravity is 2.981 (Koenig, Trotter mine) or 3.202 (Steiger, Parker shaft). Its color is pale yellow to yellowish brown, with a pearly luster on perfect cleavages.
Bementite is optically negative. Cleavage flakes on the basal cleavage show the emergence of a symmetrical biaxial interference figure with a very small axial angle, the hyperbolas barely opening as the section is revolved. The axial plane is parallel to the brachypinacoidal cleavage. X is normal to the basal cleavage. a = 1.624, b = 1.650, g = 1.650.
Composition
Bementite is a hydrous manganese silicate, generally containing some iron,
zinc, magnesium, and calcium in place of part of the manganese. Its chemical
character has been discussed by Larsen, who deduced the formula here adopted,
showed the general similarity of its composition to that of serpentine and
the similarity of its optical and other physical characters to those of certain
forms of serpentine, and suggested that it be regarded as the manganese end
member of the serpentine series. It is noteworthy in this connection that
some of the bementite and some of the manganiferous serpentine at Franklin
can be distinguished from each other only by differences of hardness and optical
characters.
|
1 |
2 |
3 |
4 |
5 |
6 |
|
| SiO2 |
39.00 |
38.06 |
38.36 |
37.93 |
39.01 |
37.03 |
| MnO |
42.12 |
53.71 |
39.22 |
53.56 |
52.65 |
54.65 |
| FeO |
3.75a |
4.94 |
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| ZnO |
2.86 |
2.93 |
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| MgO |
3.83 |
3.35 |
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| CaO |
Trace |
0.62 |
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| H2O + |
8.44 |
8.23 |
8.01 |
8.51 |
8.35 |
8.32 |
| H2O - |
0.60 |
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| Al2O3 |
0.96 |
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| Fe2O3 |
0.71 |
|||||
|
100.00 |
100.00 |
99.70 |
100.00 |
100.00 |
100.00 |
| a FeO lost and estimated by difference. |
| 1. Bementite, Trotter mine. G. A. Koenig (133), analyst. |
| 2. Same analysis recomputed to 100 percent after substituting for FeO, MgO, and ZnO their molecular equivalent of MnO. |
| 3. Bementite, Parker shaft. George Steiger (195), analyst. |
| 4. Same analysis recomputed to 100 percent after deducting Al2O3, Fe2O3, and H2O - and substituting for FeO, MgO, ZnO, and CaO their molecular equivalent of MnO. |
| 5. Composition computed according to Larsen's formula |
| 6. Composition computed according to Palache's formula. |
The water in bementite is undoubtedly constitutional, as shown by the following statement furnished by Mr. Steiger:
|
Percent of mineral |
|
| Water lost at 100° C |
0.60 |
| Water lost at 150° C |
0.24 |
| Water lost at 240° C |
0.16 |
| Water lost at low red heat |
7.57 |
| Water lost at blast heat |
0.04 |
The molecular ratio of SiO2 : MnO : H2O shown by the analyses is close to 4: 5: 3, and Palache accordingly derived the formula H6Mn5Si4O16, but it is equally close to 7 : 8 : 5, from which Larsen derived the formula H10Mn8Si7O27. The actual composition of the material analyzed is about midway between the two and is not correctly expressed by either formula, nor, indeed, can it be expressed by any formula so simple.
Occurrence
Bementite was first described by Koenig (133) and was named for C. S. Bement,
of Philadelphia. The type material from the Trotter mine consisted of stellate
masses and veinlets of friable particles embedded in calcite. But little
of the substance was found, and as the physical description was very incomplete,
bementite stood as a somewhat doubtful species.
In 1905 the author obtained from Mr. J. J. McGovern, of Franklin, specimens of bementite that he had collected at the Parker shaft in 1903, amounting to about 20 pounds. In appearance this material is unlike that from the Trotter mine, being coarsely crystallized and yielding the cleavage blocks already described. The close agreement in composition of the two sorts of material, as shown by the analyses, and the determination of its crystalline character seem to justify the acceptance of bementite as a definite mineral species.
Bementite was further established as a species by the discovery of large manganese deposits in western Washington in which bementite is a principal mineral. In their description of the deposits Pardee, Larsen, and Steiger (217) showed the essential identity of the bementite there with the Franklin mineral, and further, that caryopilite and ectropite, described from Langban, Sweden, are also identical with bementite (239).
In the course of this study of the Franklin minerals bementite has been found to be more widely distributed at Franklin than had been suspected. Many secondary veins consisting largely of calcite have a layered structure parallel to the walls, owing to the development of a brown serpentinous mineral and of white radiate willemite. Some of the brown mineral is manganiferous serpentine, and some of it is bementite. (See plate 18, B.) They have the same dense hornlike texture and conchoidal fracture, but the bementite has a hardness of 5 to 6 and a refractive index of 1.64, whereas the serpentine is soft and has an index of 1.56.
Bementite has been identified also in minute wormlike deep-brown stalactites coating crystals of tephroite and willemite in open veins. It is thought to be present in microscopic veinlets replacing the barium feldspar of the Parker shaft. (See page 59.)
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© by Herb Yeates 1997-2006.
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