Nasonite |
| Pb4(PbCl)2Ca4(Si2O7)3 |
| Hexagonal |
Forms
a(1120), m(l010), p(1011), and x(9092)?
Habit
Nasonite is ordinarily found in massive granular form, and crystals are very
rare. The only ones seen were on a specimen in the Canfield collection that
had been carefully worked out of the barite matrix. Mr. Canfield recognized
their hexagonal form and permitted the removal from the specimen of one crystal
for measurement. The first-order prism is dominant and has bright faces, broken
on several crystals by a deep, irregular vertical groove, and the prism edges
are narrowly truncated by the second order prism. The pyramid p merges
below with the prism through a curved surface, upon which a distinct facet
gave the readings determining the doubtful form x(9092). The pyramid
is sharply formed above, the habit of the crystals strongly recalling that
of witherite.
| Figure 136 |
 |
On this crystal, shown in figure 136, the following angles were measured.
Forms |
Measured |
Computed |
||||||
f |
r |
f |
r |
|||||
° |
´ |
° |
´ |
° |
´ |
° |
´ |
|
| p(1011) (2 faces) | 30 |
0 |
56 |
40 |
30 |
0 |
56 |
40 |
| x(9092) (1 face) | 30 |
0 |
81 |
36 |
30 |
0 |
81 |
40 |
From the coordinates of p was derived po = 0.8788 (G2), from which was computed the axial ratio a : c = 1 : 1.317.
The identity of the measured crystal with nasonite was assured by decisive tests for lead, chlorine, and silica made on it after the measurement of the angles was completed.
Crystals of nasonite from Langban, Sweden, which showed both prisms and the base and which were for the most part composite, with rounded faces, have recently been described by Aminoff (207). The material was mostly in lamellar masses.
Physical properties
Nasonite breaks into rudely rectangular blocks, owing to good basal and indistinct
prismatic cleavages. Its hardness is about 4, and its specific gravity is
5.425 (55 to 5.59, Aminoff).
Nasonite is white, with a greasy to adamantine luster. It is uniaxial and positive, and fragments tend to lie on a cleavage surface parallel to the optic axis. w = 1.917, e = 1.927, both ±0.005 (Larsen).
Composition
Nasonite is a silicate and chloride of lead and calcium, Pb4(PbCl)2Ca4(Si2O7)3
, analogous to ganomalite, which is Pb4(PbOH)2Ca4(Si2O7)3
, according to Penfield. The material from Franklin, the composition
of which is given in analysis 2, in the table below, was known to contain
clinohedrite, and so, in interpreting the analysis, 2.16 percent of clinohedrite,
equivalent to the ZnO found, was eliminated. Also the molecular equivalent
of CaO was substituted for those of MnO and FeO and that of Cl for OH, and
the analysis was then recomputed to 100 percent, as shown in column 3. In
column 4 the theoretical composition is given.
|
1 |
2 |
3 |
4 |
|
| SiO2 |
18.23 |
0.47 |
18.32 |
18.21 |
| PbO |
67.67 |
65.68 |
67.32 |
67.68 |
| ZnO |
0.82 |
|||
| MnO |
0.14 |
0.83 |
||
| FeO |
0.04 |
0.10 |
||
| MgO |
0.20 |
|||
| CaO |
11.29 |
11.20 |
11.59 |
11.33 |
| Cl |
2.90 |
2.81 |
3.57 |
3.59 |
| H2O |
0.24 |
0.26 |
||
| Total |
100.71 |
100.17 |
100.80 |
100. 81 |
| O = Cl2 |
0.65 |
0.53 |
0.80 |
0.81 |
|
100.06 |
99.51 |
100.00 |
100.00 |
| 1. Nasonite, Langban. Mauzelius (207), analyst. |
| 2. Nasonite, Franklin. C. H. Warren (99), analyst. |
| 3. No. 2 recomputed to 100 percent after the elimination and substitutions described above. |
| 4. Theoretical composition computed from the formula. |
Occurrence
Nasonite is one of the silicates from the Parker shaft, Franklin, described
by Penfield and Warren (179). To their description are added here only the
data and Larsen's optical data. Nasonite commonly forms the matrix of glaucochroite
crystals, and in the few specimens seen is also associated with garnet,
axinite, and barite.
The recent discovery of nasonite at Langban by Aminoff (207) adds interest to the species. He describes it as associated with a group of minerals—schefferite, native lead, apophyllite, margarosanite, and thaumasite—essentially younger than the ores and found in fissures filled with calcite. The study of the Swedish nasonite confirms in all respects the characters described for the Franklin mineral. Aminoff’s new data and analysis are given above.
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Website
© by Herb Yeates 1997-2006.
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page created: August 12, 2006 6:41 PM
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