Edenite |
Habit and occurrence
The commonest amphibole in the district is an aluminous variety, both in crystals and
granular, that ranges in color from white and light gray to bright green and dull
gray-green. Qualitative tests on several specimens showed them to contain abundant alumina
but to be low in iron and manganese; hence they are appropriately assigned to edenite.
| Figure 93 Crystal of edenite showing the forms m(110) and r(011). Franklin. |
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Edenite is not found in connection with the ore deposits but it is widely disseminated in the Franklin limestone, especially near pegmatitic contacts. Crystals of simple type from the Fowler quarry at Franklin are shown in figures 94 and 95 and in plate 10, A.
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Figure
94 Crystal of edenite showing the forms b(010), m(110), r(011), z(121), and v(231). Fowler quarry. |
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Figure
95 Crystal of edenite showing the forms b(010), m(110), r(011), i(031), p(101) and z(-121). Fowler quarry. |
Figures 96 to 99 illustrate the complex and sharply formed crystals that were found, together with aluminous pyroxene, in a pocket in limestone near the Noble mine at Sterling Hill (locality 17).
| Figure 96 Crystal of edenite showing the forms b(010), m(110), r(011), p(101), v(231) and z(121). Sterling Hill. |
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Figure
97 Plan of a crystal of edenite showing the forms b(010), a(100), m(110), r(011), i(031), t(101), p(101), k(211), v(231) and z(121). Sterling Hill. |
They are dull gray to dark grayish black, and as much as 6 inches in greatest diameter, the habit being generally equidimensional or shortened in the direction of the vertical axis.
| Figure 98 Projection on the clinopinacoid of a crystal of edenite showing the forms b(010), m(110), r(011), i(031), p(101), k(211), v(231) and z(121). Sterling Hill. |
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Figure
99 Crystal of edenite twinned on the orthopinacoid and showing the forms b(010), m(110), r(011), i(031), t(101), k(211) and v(231). Sterling Hill. |
The Canfield and Kemble collections are especially rich in these unique crystals of amphibole. A very similar occurrence of edenite, on the Munson farm east of Mine Hill, Franklin, is shown in figure 100, drawn from a single crystal in the Kemble collection.
| Figure 100 Projection on the clinopinacoid of a crystal of edenite twinned on the orthopinacoid and showing the forms b(010), m(110), r(011), i(031), t(101), p(101), k(211), v(231) and z(121). Franklin. |
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The diallage of earlier lists, described as gray to bright green, probably was edenite, so far as could be determined by specimens in old collections. Figure 101 shows the crystal type.
| Figure 101 Crystal of edenite showing the forms b(010), m(110), r(011), i(031) and p(101). Franklin. |
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Optical properties
A crystal from
the Noble mine at Sterling Hill was studied in the mineralogical laboratory
at Harvard University by Messrs. Berman and Gonyer. Edenite is optically
biaxial and positive; 2V medium (60° ±); r < v ; Z /\
c = 27° ±l° ; Y = b;
absorption, g
>b
>a . Indices
and pleochroism, a = 1.622,
colorless; b
= 1.630, light violet-blue; g
= 1.645,
light violet-blue (Berman).
Composition
The following analysis shows the composition of edenite:
| SiO2 |
47.33 |
| TiO2 |
0.42 |
| Al2O3 |
8.68 |
| Fe2O3 |
1.02 |
| FeO |
3.38 |
| MnO |
0.13 |
| MgO |
21.38 |
| CaO |
12.36 |
| Na2O |
2.84 |
| K2O |
0.86 |
| H2O - |
0.04 |
| H2O+ |
1.04 |
| P2O5 |
None |
| F |
0.69 |
|
100.17 |
|
| O = F2 |
0.29 |
|
99.88 |
The analysis shows a composition closely comparable to that of the type edenite from Edenville, N.Y., according to Dana6. The following discussion interprets the analysis in terms of the method of Berman and Larsen (279).
The relation of the composition found to the formula required by the method is as follows:
| Found: | Ca1.9(Na,K)1(Mg,Fe",Mn)5(Al,Fe''')1.5(Si,Ti)6.9O22.7(OH,F)1.3 |
| Required: | Ca2Na1(Mg,Fe")5(Al,Fe''')1Si7O22(OH,F)2 |
The type edenite from Edenville has approximately the composition Ca2Na1Mg5Al1Si7O22(OH)2 ; hence the Sterling Hill mineral is undoubtedly of the same type.
The edenite formula may be considered as related to that of tremolite in the following manner:
| Edenite, Ca2NaMg5(Al,Si)8O22(OH)2 |
| Tremolite, Ca2Mg5Si8O22(OH)2 |
Thus, edenite is equivalent to tremolite in which an atom of aluminum and one of sodium compensate the deficiency of silicon and balance the valences.
6 System of mineralogy, 6th ed., page 395 (analysis 100), 1892.
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© by Herb Yeates 1997-2006.
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