Most fowlerite crystals are square prismatic parallel to the edge between
the base and the negative prism, but some are thin-tabular parallel to
the base, to the prism M(110), or to some other plane.
Complete individuals are rare, as the crystals are commonly in parallel
groups implanted on a base of massive rhodonite; some single crystals
are 8 inches long and 2 inches square. Fowlerite is also found foliated,
in coarse granular form, either massive or friable, and in isolated rounded
grains. It is rarely twinned polysynthetically parallel to the base.
Crystal of rhodonite showing the forms c(001), a(100),
m(110), M(110), k(221), and n(221).
Fowlerite crystals commonly
have dull faces, but on a few the forms c, m, M, and n are
lustrous. All edges are apt to be more or less rounded, partly because
of obscure truncating planes but chiefly because of the apparent resorption
that affects almost all the minerals embedded in limestone and that also
causes irregular pitting on the crystal faces. Rarely the crystals are
developed in open cavities and may there be brilliant and lustrous.
Crystal of rhodonite (variety fowlerite) showing the c(001),
b(010), a(100), m(110), M(110), m(401),
k(221), r(111), l(443) and n(221).
Franklin. A, Plan; B, clinographic projection.
Crystal of rhodonite (variety fowlerite) showing the forms c(001),
b(010), a(100), m(110), M(110), m(401),
K(403), ), q(221), k(221), r(111)
and n(221). Franklin. A, Plan; B, clinographic projection.
The cleavage parallel
to the prisms m(110) and M(110) is perfect, and that parallel
to the base is less perfect but good. A parting parallel to the brachypinacoid
is very good, and there are traces of parting parallel to the macropinacoid.
The hardness is 5.5 to 6, and the specific gravity is 3.4 to 3.68. The
crystals are very brittle, owing to innumerable minute cracks. The color
is bright to pale rose-pink, brownish red, or gray; it is blackened on
weathered surfaces and quickly bleached to dirty pinkish white or gray
by direct sunlight. The luster is vitreous and the mineral is translucent
or rarely, in small crystals, transparent.
Fowlerite is optically
biaxial and positive; 2V moderately large; dispersion easily perceptible;
crossed dispersion rather strong; a
= 1.716, b
= 1.720, g
= 1.732. These optical data are for the fowlerite of analysis 4 (Larsen,
221). The orientation of the axial ellipsoid of fowlerite has not been
determined but is probably similar to that of rhodonite, which has been
determined by Hey (269). (See
page 69, under bustamite.)
Fowlerite is a manganese silicate with considerable amounts of calcium,
iron, magnesium, and zinc in place of part of the manganese. The zinc
is peculiar to the rhodonite from the district and justifies the early
local name of fowlerite for the zinciferous variety of rhodonite. Most
of the early analyses were made on altered material and are of little
value; the only trustworthy ones are given below.
[Analyses of fowlerite]
The analyses show a considerable
range in the extent to which other elements take the place of manganese.
Analyses 4 and 7 are interesting especially as showing compositions nearer
to that of bustamite. Probably analyses of more material would show other
intermediate compositions. The material for analysis 4 consisted of the
centers of fowlerite crystals that were coated with a parallel growth
of bustamite in crystallographic continuity with the core.
At Franklin large masses of fowlerite are embedded in calcite, generally
near the boundary of the ore body and as a rule demonstrably at or close
to the contact of an igneous intrusion. The enclosing calcite is in general
coarsely crystalline, white or bluish, and commonly in veinlike masses
intersecting the ore body and more or less completely interpenetrated
by fowlerite, alone or with franklinite, willemite, and zincite. Fowlerite
is also found not infrequently as isolated grains in the mass of the pegmatite.
The great abundance of
the mineral where pegmatite is prominent, as at the Parker shaft and the
Trotter and Buckwheat mines, and its scarcity elsewhere in the mines confirm
the conclusion that fowlerite is exclusively a metamorphic mineral formed
through the influence of the pegmatite intrusions. In the mine workings
opened near the walls of the ore body blasts sometimes stripped surfaces
square yards in extent covered with this beautiful pink mineral. Groups
of crystals, such as those preserved in the Bement, Kemble, and Lang collections,
taken from such localities, represent the finest known development of
this mineral. Most specimens of rhodonite seen in collections have been
freed from the enclosing limestone by treatment with acida process
that is very destructive to the beauty of the mineral, as it is itself
attacked slightly and more or less decolorized. Crystals freed from the
matrix by a chance blow of the hammer or by the laborious method of scraping
away the calcite bit by bit with a steel point are in every way finer
in appearance. The splendid group in the Bement collection, illustrated
in plate 9, A, was so prepared, and
plate 9, B, shows some of the best of
Mr. Hancock's work.
Where developed in cavities
the crystals are generally small, of a clear transparent pink color, and
brilliantly lustrous. Such a crystal is shown in figure 91, and combination
10, flattened parallel to the prism M(110) (figure 90), is from
a similar setting.
Crystal of rhodonite (variety fowlerite) tabular parallel to the
prism M (110), showing the forms c(001), b(010),
a(100), m(110), M(110), ), f(130),
p(111), q(221), k(221), i(441),
r(111), l(443) and n(221). Franklin.
A, Plan, and B, clinographic projection of a tabular crystal of
rhodonite showing the forms the c(001), m(110),
p(111), q(221), k(221) and r(111).
The most commonly associated minerals
in such places are yellow garnet, in crystals or massive, and manganoaxinite.
Calcite and barite also occur with it.
The fowlerite crystals coated
with bustamite in parallel growth, described by Larsen and Shannon (221),
are apparently also a vein product. They are associated with manganoaxinite,
barite, willemite, and ganophyllite.
At Franklin fowlerite was found
by the earliest mineralogical explorers, has always been abundant, and
unlike the other common minerals of the deposit is still found in specimens
equal to or better than those taken from the earlier workings. On the
other hand, it was never found in abundance at Sterling Hill, and only
two or three poor specimens from there were seen in all the collections
The alteration of fowlerite to a manganiferous serpentine is described
117. All stages of the change were noted, from crystals showing only
small spots of altered material to true alteration pseudomorphs in which
the whole mass was changed, without loss of crystal form, into harsh,
dirty-white serpentinous material. No special study of this process was
attempted. The name "hydrorhodonite" is given locally to this
alteration product, some of which is very hard as the result of cementation
by silica set free during the alteration.
Under the name "dyssnite"
Von Kobell quotes a description and analysis given by Thomson (23) of
an oxidized rhodonite, which he styled "sesquisilicate of manganese"a
scaly black, metallic-looking substance. Thomson (32) afterward withdrew
this analysis as untrustworthy and gave another, of supposedly fresher
material, under the same name "dyssnite." These analyses are
not exact enough for determination of the kind of chemical changes that
the fowlerite had undergone.
The first analysis of rhodonite from Franklin was made by Thomson (23),
who called it "ferrosilicate of manganese." Other early names
used for it are "siliceous oxide of manganese" (Fowler, 21)
and "manganesian feldspar" (Torrey, 11). The first analyses
were made on very impure material.
The name "fowlerite,"
first found in Shepard's "Mineralogy" (28), was bestowed on
the mineral by Nuttall in the early twenties of the past century, in honor
of Dr. Fowler, of Franklin, one of the first collectors of the local minerals.
The probable identity of the mineral with rhodonite was first pointed
out by Hermann (45) and afterward by Dauber (70), who measured crystals
and showed them to be triclinic. The name "fowlerite" is still
in use for the zinciferous variety of rhodonite.
page created: August 12, 2006 6:50 PM