Allanite

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.]

**Combinations on crystals of allanite**
	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. Bx_a /\ 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.

**Analysis of allanite**
(T. S. Hunt (83), analyst)
	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.


	Website © by Herb Yeates 1997-2001.
	This page created: January 12, 2001 5:40 PM