Leucophoenicite |
| H2Mn7Si3O14 |
| Monoclinic |
Forms
c(001), b(010), a(100), m(110), s(120),
e(101), t(102), x(103), r(101),
i(102), y(103),
z(104), o(011), f(012),
p(111), l(121), n(121),
u(122), j(122), k(211),
d(123), h(123), and q(124)
[Combinations on crystals of leucophoenicite]
Habit
Leucophoenicite is commonly found in isolated crystalline grains or in
massive granular form. Crystals are rare and are generally of epidote
habit, more or less elongated parallel to the orthoaxis, and with a striated
orthodome zone. Some, however, are tabular parallel to the orthopinacoid
or to an orthodome. The crystals are generally minute, and many are twinned
on the base, either as contact or as interpenetrating twins, and with
numerous lamellae in parallel growth.
| Figure
151 One end of a crystal of leucophoenicite prismatic parallel to the orthoaxis, showing the forms c(001), b(010), a(100), s(120), e(101), r(101), i(102), y(103), o(011), l(121), and n(121). Parker shaft, Franklin. |
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Figure
152 Projection on the clinopinacoid of twin crystals of leucophoenicite twinned on the base and showing the forms c(001), b(010), a(100), e(101), t(102), r(101), i(102), o(011), l(121), n(121), u(122), d(123), and h(123). Parker shaft, Franklin. |
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Figure
153 |
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| Figure
154 Clinographic projection of the crystal of leucophoenicite illustrated in figure 153, showing the prismatic development parallel to the orthoaxis. Franklin. |
The crystallography of leucophoenicite was first described by the author (195), in 1910 and more fully (257), in 1927, Penfield (179), who established the species in 1899, having been unable to determine on his material even the crystal system. The first crystals measured came from a specimen in the collection of Mr. Canfield, who most kindly consented to sacrifice part of it for the purpose. Some 10 crystals, showing combinations 1 to 8, proved measurable, but many were fragmentary, and none were of first-class quality. The position chosen was that which gave the simplest symbols and at the same time brought the plane of twinning and cleavage into the position of the basal pinacoid. No interpretation of the highly peculiar assemblage of forms showed the slightest resemblance to the form series of any member of the humite group, to which leucophoenicite is chemically related.
Physical properties
The cleavage is imperfect but is distinct parallel to the basal pinacoid.
The hardness is 5.5 to 6, and the specific gravity 3.848. The color ranges
from brown through light, purplish-red to raspberry-red (whence the name).
The mineral is faintly pleochroic—rose-red for vibrations parallel
to the base and colorless for those normal to it, and it is also colorless
in thin section. It is negative; 2V= 74° ±5' ; r > v (slight);
a = 1.751,
b = 1.771,
g = 1.782,
all ±0.003 (Larsen).
Composition
Leucophoenicite is a basic silicate of manganese, of closely similar type
of composition to humite but containing no fluorine and with manganese
in place of magnesium. The Franklin material contains more or less calcium,
zinc, magnesium, and ferrous iron in place of about an eighth of the manganese,
and practically negligible amounts of alumina, soda, and potash. The analyses
differ only in the relative amounts of the several bivalent oxides, and
analysis 3 shows only about a third of the normal amount of water. With
that exception the analyses give very closely a molecular ratio of RO
: SiO2 : H2O = 7 : 3 : 1, in which RO is chiefly
MnO. The corresponding formula is H2Mn7Si3O14,
or, stated in molecular form, Mn(OH)2.3Mn2SiO4.
The close chemical similarity of this formula to that of humite, Mg(OH,F)2.3Mg2SiO4,
pointed out by Warren (179), is striking, but in spite of it, as stated
above, there seems to be no crystallographic similarity between the two.
Occurrence
Leucophoenicite is fairly abundant in specimens from the pneumatolytic contact
zone in the Parker shaft. It is found as grains and imperfect crystals intimately
associated with bright-green willemite and sparse crystals of brown vesuvianite.
Its color makes it conspicuous and rather attractive, but it is easily mistaken
in some of its color shades for phases of willemite and garnet, and it was
at first taken to be clinohedrite. Besides the material from the original
locality, leucophoenicite was identified in 1906 by Penfield in specimens
in the Brush collection that had been found in the Buckwheat mine many years
before. In those specimens it is in crystalline grains in calcite with green
willemite and is quite similar in appearance to material in the type specimens,
but it had been over looked or mistaken for another substance (private communication
to the author). It was also found in the complex specimens from the Buckwheat
mine that contain pyrochroite (see
page 50), in which it is partly massive and mixed with franklinite and
dull-pink garnet, and partly in free crystals, as described above.
More recently two interesting finds have been reported by H. H Hodgkinson, of Franklin. One was a small cavity in ore, an opening in a vein consisting of leucophoenicite and willemite in granular mixture. The cavity is lined with pale-yellow drusy garnet on whose surface are implanted crystals of leucophoenicite and willemite and a single white scalenohedron of calcite. The willemite is pale green, of brilliant luster and long prismatic habit, with the complex form of combination 14 (page 82). The leucophoenicite is in clear-red crystals, in striking contrast to their background. They are slender prismatic forms, both simple and twinned, the longest being doubly terminated, about one-third of an inch long, and showing the forms of figure 153. The twin shown in figure 155 and combinations 10, 11, and 12 came from this specimen.
| Figure
155 Projection on the clinopinacoid of a crystal of leucophoenicite twinned on the base, showing the forms c(001), a(100), s(120), e(101), x(103), r(101), i(102), y(103), z(104), u(122), and k(211). Franklin. |
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The crystals have suffered somewhat from etching, and the angle readings obtained were poor, but they sufficed to establish the forms—the orthodome z(104) and the pyramids u(122) and k(211) being characteristic.
Leucophoenicite was also found by Mr. Hodgkinson in the north end of the mine near the hanging wall of the west leg of the ore body, within 2 feet of a pegmatite dike, in a continuous seam with swells and pinches, the swells making vugs in which the crystals had formed. The cavities have walls of layered ore containing much franklinite, which, near the margins of the cavities, is in cubic crystals. The walls of the cavities are lined with gray calcite merging inward to pale rhodochrosite, poorly crystallized in parallel groups of rhombohedrons. On the carbonates is a coating of silky, felted sussexite, commonly in a thin closely adhering film. Massive dull-brown leucophoenicite forms a central mass 4 inches across, crystallized toward the center, either in slender, plate-like crystals, shown in figure 156, their broad surfaces deeply striated by twinning, with bright surfaces of the base or basal cleavage; or in isolated stouter and more brilliant crystals, like figure 157.
| Figure
156 Unsymmetrical crystal of leucophoenicite twinned on the base, showing on one part the forms a(100) and b(010) and on the other part the forms c(001), a(100), i(102), and y(103). Franklin. |
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Figure
157 Crystal of leucophoenicite showing the c(001), b(010), a(100), m(110), s(120), e(101), r(101), i(102), y(103), u(122), q(124), p(111), and j(122). Franklin. A, Projection on the clinopinacoid; B, Clinographic projection. |
The latter are clear vivid pink and the plates are clear to opaque dull brown. Some of the platy crystals are aggregated in fan-shaped groups rising from the massive matrix.
These specimens have added much to our knowledge of the crystallography of leucophoenicite and are the most attractive yet found of that peculiar mineral.
There has also been found a striking vein occurrence of leucophoenicite with willemite, the vivid color contrast with the green willemite being typical.
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© by Herb Yeates 1997-2006.
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