Allanite |
| (Ca,Fe)2(AlOH)(Al,Ce,Fe)2(SiO4)3 |
| Monoclinic |
Forms
a(100), c(001), m(110), u(210), m’(102), e(101),
s(103), i(102), r(101),
l(201), o(011), d(111), n(111), and w(211).
[Web Ed. note: The form m’ appeared as a
lowercase Germanic ‘m’ in the original.]
| Forms | Illustrations |
|
| 1 | a, c, m | Eakle (150), figure 1 |
| 2 | a, c, m, e, l, d, n, | Eakle (150), figure 2 |
| 3 | a, c, m, u, m’, e, r, l, d, n | Figure 146. Eakle (150), figure 3 |
| 4 | a, c, m, u, e, m’, i, r, l, d, n, w | Eakle (150), figure 4 |
| 5 | a, c, m, e, i, r, l, o, d, n | Eakle (150), figure 5 |
Habit
The crystals are commonly tabular parallel to the orthopinacoid, and many are elongated in
the direction of the orthoaxis. The faces are dull and pitted and give poor reflections on
the goniometer, but the forms listed were well established by numerous measurements.
The cleavage is very imperfect parallel to the orthopinacoid and to the base and is a little better nearly parallel to the unit prism but departs about 6-½° from parallelism. The specific gravity is 3.84 (Hunt). The mineral is coal-black and opaque, but in thin section it is transparent, brown, and strongly pleochroic; X yellow, Y dark red-brown, Z grayish brown. The absorption is strong of rays parallel to Y. Refractive index about 1.74 and birefringence weak. The axial plane is parallel to the clinopinacoid. Bxa /\ c = 36° (about), in acute angle b.
Composition
The only analysis of allanite from Franklin is given below. It is that of a typical
allanite, which is a hydrous silicate of aluminum and calcium with more or less iron,
manganese, and a few of the rare-earth metals. The material analyzed probably contains
some ferrous iron and also a little of some other rare earth, such as Pr2O3
or Nd2O3.
Percent |
|
| SiO2 | 30.20 |
| Al2O3 | 13.05 |
| Fe2O3 | 18.25 |
| Ce2O3 | 16.60 |
| La2O3 | 6.90 |
| CaO | 11.76 |
| MgO | 1.70 |
| MnO | Trace |
| Volatile | 1.30 |
99.76 |
Occurrence
Allanite is found at Franklin only in the pegmatite. It was first recognized by Jackson
(51), who found it on the dump of one of the old iron mines. It can still be found in
imperfect black crystals in the pieces of pegmatite on several of those old dumps. This
was the material analyzed by Hunt.
In sinking the Trotter shaft on Mine Hill much pegmatite was encountered, and in places, especially where the coarse pegmatite was in contact with either limestone or zinc ore, allanite was abundant. The crystals are tabular, and many are thin and highly brittle, so that they are extracted from the matrix with much difficulty. Plates as much as 1.5 by 3 inches were found, but few complete crystals are more than an inch in their greatest dimension. The crystals were described by Eakle (150), and the foregoing description has been taken from his paper. Plate 16, B, shows a crystal embedded in the pegmatite, and figure 146, after Eakle, shows the commonest combination.
| Figure 146 Crystal of allanite showing the forms c(001), a(100), m(110), u(210), m’(102), e(101), r(101), l(201), d(111), and n(111). Franklin. |
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Website
© by Herb Yeates 1997-2006.
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