Habit
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.

Figure 87 Crystal of rhodonite showing the forms c(001), a(100), m(110), M(110), k(221), and n(221). Franklin.

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.

Figure 88 Crystal of rhodonite (variety fowlerite) showing the c(001), b(010), a(100), m(110), M(110), m(401), f(201), r(401), k(221), r(111), l(443) and n(221). Franklin. A, Plan; B, clinographic projection.
	Figure 89 Crystal of rhodonite (variety fowlerite) showing the forms c(001), b(010), a(100), m(110), M(110), m(401), f(401), r(201), 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.)

Composition
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.

Occurrence
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 acid—a 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.

Figure 90 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.
	Figure 91 A, Plan, and B, clinographic projection of a tabular crystal of rhodonite showing the forms the c(001), m(110), M(110), r(401), f(201), p(111), q(221), k(221) and r(111). Franklin.

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 studied.

Alteration
The alteration of fowlerite to a manganiferous serpentine is described on page 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.

Historical notes
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.