Margarosanite |
| PbCa2(SiO3)3 |
| Triclinic |
Crystal form
Margarosanite is found at Franklin only in lamellar masses composed of thin
plates packed closely together and showing individually a rhombic outline
that is due to cleavage. The crystal form is known, however, from the description
by Flink (216) of crystals from Langban. At Langban the crystals are found
with schefferite, apophyllite, calcite, nasonite, and thaumasite, in veins
cutting dolomite. Most of the margarosanite is massive, but in vugs a few
slender crystals were found with faces somewhat curved or striated, yielding
poor measurements. The habit, as shown by figure 92, is pyroxenelike, and
the mineral is regarded by Flink as an independent species of the pyroxene
group, a conclusion in which the author fully concurs.
| ( a : b : c = 0.8442 : 1: 1.2838; po = 1.4943, qo = 1.0097; a = 74° 37', b = 129° 30', g = 101° 6' ; l = 100° 52', m = 51° 49', v = 88°10' ) |
Form |
Computed |
Measured |
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f |
r |
f |
r |
|||||
° |
´ |
° |
´ |
° |
´ |
° |
´ |
|
| c(001) | 106 |
48 |
40 |
43 |
||||
| a(100) | 88 |
10 |
90 |
0 |
*88 |
10 |
90 |
0 |
| b(010) | 0 |
0 |
90 |
0 |
||||
| m(110) | 54 |
42 |
90 |
0 |
*54 |
42 |
90 |
0 |
| n(110) | 122 |
47 |
90 |
0 |
119 |
30 |
90 |
0 |
| d(011) | 37 |
15 |
53 |
42 |
||||
| e(034) | 146 |
34 |
56 |
14 |
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| Angles marked with an asterisk (*) used in computing crystallographic elements. |
Flink's measurements were not two-circle measurements. The computation of the crystallographic elements was based on the angles in the table marked with the asterisk (*) and on the following measurements, the first of which has been amended from the published figure after correspondence with Dr. Flink: (001) /\ (010) = 100° 52' = l, (001) /\ (100) = 51° 49' = m, (011) /\ (010) = 50° 06'.
Physical properties
The cleavage is perfect parallel to the brachypinacoid and is good parallel
to the base and to the macrodome (504) (Flink). The hardness is 2.5 to 3,
the specific gravity is 3.991, and the mineral is colorless and transparent,
showing a pearly luster on cleavage surfaces. Under the iron-arc spark it
fluoresces with a rather lively pale-violet tint.
Margarosanite is optically negative; 2V = 83° ±5°, r > v (perceptible); a = 1.729 ±0.005, b = 1.773 ±0.003, g = 1.807 ±0.005 (Larsen). The plane of the optic axes is nearly normal to the best cleavage, with an axis revolving just outside the field of vision.
Composition
Margarosanite is a silicate of lead and calcium with a derived formula Pb(Ca,Mn)2Si3O9.
|
1 |
2 |
3 |
|
| SiO2 |
33.71 |
34.72 |
34.95 |
| PbO |
43.50 |
41.74 |
43.29 |
| CaO |
21.73 |
20.28 |
21.76 |
| MnO |
1.14 |
2.17 |
|
| MgO |
0.20 |
||
| BaO |
0.69 |
||
| H2O |
0.58 |
0.10 |
|
|
100.68 |
99.90 |
100.00 |
| 1. Margarosanite, Franklin. W. M. Bradley (208), analyst. |
| 2. Margarosanite, Langban, Sweden. G. Flink (216), analyst. |
| 3. Composition computed from the derived formula. |
Pyrognostics
Margarosanite fuses in the oxidizing flame with some difficulty, the fragment
assuming an amethyst color, but in the reducing flame it fuses easily and
quickly at about 2 to an opaque grayish glass and colors the reducing flame
pale azure-blue. With fluxes on charcoal it forms a metallic globule of
lead and a lead oxide coating, and it gives manganese colors in soda and
borax beads. It is decomposed by nitric acid, yielding separated silica.
Occurrence
Margarosanite was one of the many species found at the Parker shaft in 1898,
along with barite, garnet, hancockite, roeblingite, nasonite, franklinite,
willemite, axinite, datolite, and manganophyllite.
It was partly investigated by Penfield and Warren (179); the study was later completed and the name given by Ford and Bradley (208). The name is derived from Greek words meaning a pearl and a tablet or board, in reference to its pearly luster and lamellar structure.
In 1919 Flink (216) announced the occurrence of margarosanite at Langban in Sweden.
| Figure 92 Crystal of margarosanite from Langban, showing the forms c(001), b(010), a(100), m(110), n(11), d(011) and e(034). Figure drawn from description of Flink (217). |
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Website
© by Herb Yeates 1997-2006.
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page created: August 12, 2006 8:17 PM
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