Physical character
The cleavage is indistinct parallel to the base and to the prism a(1120) but is not always evident unless the fracture is carefully directed. The fracture is irregular to conchoidal, the hardness is 5.5, and the specific gravity is 3.89 to 4.19. The luster is vitreous in the purest crystals to resinous in the colored varieties. The color is extremely diverse; the most perfect transparent crystals are colorless to pale green, bright green, or wine-yellow; the opaque crystals and the massive mineral are snow-white, gray, apple-green, all shades of yellowish green and greenish yellow, flesh-red, mahogany-red, and dark brown; the weathered surfaces are not uncommonly black.

Willemite is optically uniaxial and positive. The refractive indices, determined on a prism cut parallel to the vertical axis of a clear wine-yellow crystal, are, for sodium light, w = 1.694, e = 1.725; for lithium light, w = 1.689, e = 1.718. The unusually pure willemite of analysis 21 gave the following indices for white light by the immersion method: w = 1.691, e = 1.719.

Willemite is generally fluorescent, and some specimens are strongly phosphorescent with a green color, but the purest white willemite, like troostite, gives little or no reaction. The so-called "black willemite" does not fluoresce.

Chemical composition
Willemite is an orthosilicate of zinc, containing more or less manganese, iron, and magnesium in place of part of the zinc. The published analyses of the mineral are given below.

Computation of the molecular ratios of the analyses shows that the departure from the orthosilicate formula is hardly noticeable. Manganese and zinc are interchangeable in the mineral in all proportions up to about 12 percent of manganese, and other elements are rarely present in more than insignificant amounts.

It is noteworthy that the radiated fibrous material contains less manganese (see analysis 15) than any other willemite analyzed except the transparent crystals (see analysis 21), which contain only 0.12 percent of MnO. It seems probable that this small amount of what may be regarded as an impurity in the mineral is the cause of the extreme sensitiveness to ultraviolet light shown by the fibrous white willemite. Some specimens of the fibrous variety remain phosphorescent 15 minutes or more after exposure to the iron-arc spark or to the light of burning magnesium. It is well known that many compounds that are not phosphorescent if either absolutely pure or very impure are strongly excited if they contain but a trace of impurity.

Occurrence
At Franklin willemite is a constant and abundant constituent of the whole ore body and is that one of the valuable zinc ores from which spelter is chiefly made. It is normally in the form of greenish, yellowish, or reddish rounded grains intimately mixed, generally in layers, with franklinite, calcite, and a little zincite. Crystals of willemite of definite form are practically unknown in the primary ore. Crystals of great variety of color and habit are, however, found in the various secondary deposits associated with the main ore body.

In the pneumatolytic zone of the Parker shaft willemite was found in transparent crystals of fine green color, intimately associated with nasonite, glaucochroite, leucophoenicite, datolite, and the other silicates peculiar to that part of the mine. Some crystals are an inch long, but most of them are minute. Their prism faces are brilliant, but many are marked by natural etch figures of a characteristic asymmetric form. Terminal faces are commonly less brilliant but on some specimens are of the most perfect quality, and on such crystals the many forms listed herein have been seen.

Figure 116 Doubly terminated rhombohedral crystal of willemite showing the forms a(1120), e(0112), r(1011), and x(3121). Franklin.
	Figure 117 Crystal of willemite showing the forms a(1120), m(1010), r(1011), n(0221), k(4132), x(3121), y(2131), d(1232), D(1322), and q(1341). Franklin. Plan; B, clinographic projection.

Masses of transparent, glassy yellowish-green willemite, weighing a pound or more and measuring several inches through, were mined in the Parker shaft. Clear portions of this material furnished handsome cut stones, although much of it was marred by inclusions of franklinite in the form of minute octahedrons.

Figure 118 Crystal of willemite showing the forms a(1120), m(1010), r(1011), n(0221), o(5143), x(3121), and y(2131). Franklin. A, Plan; B, clinographic projection.
	Figure 119 Crystal of willemite showing the forms a(1120), m(1010), r(1011), x(3121), t(13.5.-8.3), and j(7341). Franklin. A, Plan; B, clinographic projection.

Similar beautifully crystallized willemite of a clear, deep-green color has been found at Franklin during recent years in a number of associations. Most of these specimens were found on the picking table, and the part of the ore body from which they came is unknown. The willemite is generally in open secondary veins and was the latest mineral to form. The crystals are generally minute but show complex groups of forms.

Figure 120 Crystal of willemite showing the forms c(0001), a(1120), u(2113), ), e(0112), r(1011), n(0221), x(3121), y(2131), and D(1322). Franklin. A, Plan; B clinographic projection.
	Figure 121 Crystal of willemite showing the a(1120), b(2243), r(1011), k(4132), x(3121), y(2131), A(5231), and d(1232). Franklin. A, Plan; B, clinographic projection.

Thus willemite was found with the crystals of franklinite, described by Phillips (211), associated with rhodonite and talc (see page 46), with crystals of tephroite together with garnet, calcite, friedelite, and bementite (see page 77), and with leucophoenicite and sussexite (see page 105).

Figure 122 Short prismatic crystal of willemite showing the forms c(0001), a(1120), r(1011) and e(0112). Franklin.

In 1926 Palache and Berman (251) described pure white willemite with the form of figure 123, remarkable as showing for the first time the negative rhombohedron e(0112) as the dominant terminal face.

Figure 123 Short prismatic crystal of willemite showing the forms c(0001), a(1120), e(0112) and s(1123). Franklin. A, Plan; B, clinographic projection.

In the same paper the crystals illustrated in figure 124 were described. They are white, transparent, and of thin tabular form with scarcely a trace of prism faces. They were associated with axinite, rhodonite, cahnite, hedyphane, and barite.

Figure 124 Hexagonal tabular crystal of willemite showing the forms c(0001), a(1120), e(0112) and r(1011). Franklin. A, Plan; B, clinographic projection.

In 1928 Palache described the most beautiful crystals of willemite yet found at Franklin. They came from a vein that was open for at least part of its length but was probably little more than a crevice. Coating the vein wall is a crust of drusy yellow garnet and on it are pink hodgkinsonite crystals of unusual habit (see page 109), bluish tephroite crystals with tips blackened by manganese oxide, crystals of clear or snow-white barite, and a few microscopic needles of a vanadate, believed to be a manganiferous descloizite. Many of the specimens from this vein are studded with willemite crystals of peculiar beauty. They are prisms of absolute transparence, colored a fine uranium-green, the largest 0.4 inch long and 0.08 inch in diameter. Their habit is dominated by a stepwise development of the third-order rhombohedron x(3121) in combination with the second-order prism. Figure 125 shows in part the deeply striated appearance of these crystals.

Figure 125 Tapering prismatic crystal of willemite showing the forms a(1120), r(1011), x(3121), y(2131), and k(4132). Franklin. A, Plan; B, clinographic projection; C, photograph.

So clear is the material that striations on the back, of a crystal are plainly visible through its thickness. Some crystals, attached by a prism face, are doubly terminated, giving the effect of enormously elongated rhombohedrons. As shown in the plan, figure 125, A, the upper part of the crystal is bounded by three alternate faces of a, which step down with x and become smaller as the other three faces of a become larger, and in the middle of the crystal the six faces of the prism are in equal development. In figure 126 is shown in plan the distribution of faces on the more highly modified crystals, where the rhombohedral aspect is not so pronounced.

Figure 126 Plan of a crystal of willemite showing the forms a(1120), s(1123), f(4223), e(0112), g(0115), r(1011), k(4132), x(3121), y(2131), d(1232), D(1322), and q(1341). Franklin.

Among the many specimens of crystallized willemite acquired by the Harvard Mineralogical Museum with the Stanton collection was one group of colorless crystals implanted on rhodonite, distinguished by their extraordinary complexity of form. The prism zone is rounded and vertically striated, and the terminal forms are dominantly flat, as shown in figure 127.

Figure 127 Crystal of willemite showing the forms c(0001), a(1120), m(1010), F(3140), u(2113), s(1123), b(2243), r(1011), e(0112), n(0221), x(3121), y(2131), d(1232), and H(134y4). Franklin. A, Plan; B, clinographic projection.

Many phases of crystallized willemite were found in the contact zone of the Trotter mine and are preserved in collections. Most of them are due to recrystallization of the ore under the influence of pegmatite intrusions.

Dull red-brown crystals with broad base, an inch high and of slightly greater diameter, embedded in calcite with good crystals of franklinite are typical. Similar crystals with small additional faces of (1011) and of a peculiar yellow-green color, implanted on massive ore and projecting into a veinlike mass of calcite, were also found. Great masses of doubly terminated green crystals, so large that they were for the most part not recognized as distinct crystals (Canfield), were found near the bottom of the north end of the Trotter mine.

The most peculiar find of willemite at this mine was in the form of long, slender clear-yellow prisms, whose various terminations are shown in figures 128, 129, and 130.

Figure 128 Slender prismatic crystal of willemite showing the forms c(0001), a(1120), u(2113), s(1123), and e(0112). Trotter mine.
	Figure 129 Slender prismatic crystal of willemite showing the forms a(1120), u(2113), and s(1123). Trotter mine.
Figure 130 Slender prismatic crystal of willemite showing the forms a(1120), and u(2113). Trotter mine.

The crystals, which are nearly 3 inches long and less than a quarter of an inch thick, lie firmly embedded in a brownish-black claylike mass, apparently the filling of a narrow crevice. Plate 13, A, shows one of these specimens. The long axes of the crystals lie in the plane of the vein; some of them abut upon one another and some are terminated. With them in the matrix are rhombs of calcite and flattened octahedrons of white sphalerite. The willemite crystals show distinct basal cleavage, but all faces are dull and give poor readings.

In the Taylor and Buckwheat mines, especially, were numerous secondary carbonate veins transverse to the ore body, which in many places contained crystallized willemite. In an opening in such a vein in the wall of the Buckwheat mine Mr. Canfield found a number of transparent topaz-yellow crystals, the largest 2 inches long and nearly as broad. Terminal faces are wanting, but the crystals have a high luster, and from some of the material beautiful gems have been cut. One of these crystals, supplied by Mr. Canfield, was used in measuring the refractive indices of the mineral.

Similar material, which shows two periods of crystal growth, was found, seemingly in great abundance, in the open-cut quarry (the old Buckwheat mine) in 1917. The first period produced stout, rounded, and commonly very large crystals of the habit of figure 130; on the ends of some of these is developed a cluster of needlelike crystals in parallel position, each crystal terminated by faces of the most complex type, with rhombohedrons of all three orders. From these crystals, also, gems of great beauty have been cut.

The crystal figured by Penfield (156) was also found in this mine. Almost transparent pale-green prisms embedded in calcite were attached to massive ore, clearly part of a transverse vein.

In the Losey and Hancock collections were specimens from the Hamburg mine, with slender beryl-colored crystals of willemite, some 4 inches long and not more than an eighth of an inch across, embedded in pale rose-tinted manganiferous calcite, from which, by most careful chiseling, the prisms had been freed.

Fibrous forms of willemite—white, pale green, and reddish, the fibers parallel or radiate—are confined to the transverse carbonate veins, layers of willemite with fibers normal to the vein wall generally alternating with carbonate layers. Such veins were seen in abundance in the Buckwheat open cut in 1905. This fibrous form of willemite is prized for its high sensitiveness as a luminous screen for ultraviolet rays, as it gives a higher luminosity than the massive forms. Plate 14, A, B, and D, shows this mode of occurrence.

At Sterling Hill also willemite was widely distributed in granular form throughout the ore body. It appears, however, to have been neither so abundant nor so regularly distributed as at Franklin. The more characteristic mode of occurrence there was in well formed crystals isolated in the limestone or mingled with crystals of franklinite. It was to these reddish or flesh-colored crystals that the name "troostite" was applied. Figures 131 and 132 and plate 13, B, show their ordinary forms.

Figure 131 Crystal of willemite, variety troostite, showing the forms a(1120), r(1011), and e(0112). Sterling Hill.
	Figure 132 Crystal of willemite, variety troostite, showing the forms a(1120), and r(1011). Sterling Hill.

The prismatic crystals reach a length of 10 inches with a diameter of 4 inches or more.

From the deep workings of the Sterling Hill mines were obtained a few specimens of richly colored green willemite showing well-developed crystals embedded in pale pink manganiferous calcite. These specimens, preserved in the Canfield collection, include the unique crystal (combination 23) first figured by Penfield (156), which is terminated by the third-order rhombohedron x(3121) only. This crystal was afterward excavated more fully by Canfield and the lower end exposed. Canfield (192) gives a figure that shows a striking hemimorphism. In the absence of corroboration on other crystals the importance of this hemimorphic development in its bearing on the true symmetry of willemite crystals must remain doubtful.

Only two finds of material of this sort are certainly known from Sterling Hill, and there is no doubt that such types of crystallization are less common than at Franklin.

Alteration
Beyond a surface blackening from separation of manganese oxide, not infrequently seen on troostite crystals, willemite rarely shows signs of alteration. However, from the calamine deposit at Sterling Hill were obtained crystals of troostite covered or completely replaced by a sheath of glassy needles of calamine blackened by manganese oxide. It is highly probable, in view of the well-preserved specimens of this sort in several collections, that much of the calamine of that great deposit was formed at the expense of willemite. Specimens of massive willemite that were found in the Buckwheat mine showed a gradual marginal alteration into a carbonate of pinkish-white color. This change is, however, not a common one, and indeed, considering the easy solubility of willemite in acids, its stability is surprising.

Historical notes
Willemite was first recognized as a mineral species by the Philadelphia mineralogists Vanuxem and Keating (17), who published a full and accurate description of its physical and chemical characters under the name siliceous oxide of zinc. Thomson (23) followed this description with an incorrect analysis which made it out to be a silicate of manganese, and his reputation as a chemist served to fix the blunder on the discoverers. To the supposed manganese silicate Shepard (29) gave the name "troostite", and about that time (1830) Levy discovered and named the willemite of Moresnet, Belgium. Later investigators, Delesse (42), Hermann (45), Dana (46), and Wurtz (50), discovered Thomson's mistake and established the essential identity of the mineral with Levy's willemite.

The name "troostite" continued to be used, however, for the dark manganiferous varieties, especially the stout crystals from Sterling Hill that were supposed to have a different cleavage from that of willemite. Hintze, in his Handbuch der Mineralogie, maintains this difference, but no sharp distinction is possible, and Dana, in the "System of mineralogy", has quite properly merged the two.

The trirhombohedral symmetry of willemite was first shown by Penfield (156), being established in part by crystals from the Franklin district. Tephrowillemite, a name given by Koenig (137) to a brown manganiferous willemite supposed to be transitional to tephroite, is to be regarded simply as a synonym for troostite.