Habit
Pyrochroite forms rhombohedral crystals with a small base, which are pseudocubic, as the rhombohedral angle (95°) is close to 90° ; prismatic crystals terminated by the base alone or by the base and rhombohedrons; scalenohedral crystals approximating the unit rhombohedron in form; and foliated masses filling thin veinlets.

Physical properties
Pyrochroite is colorless to pale pink, generally changing rapidly on exposure to the air, first to olive-green and then to opaque black. The cleavage is basal and perfect. Cleavage flakes, when unaltered, show a uniaxial optical figure.

Composition
Pyrochroite is manganous hydroxide. The following analyses of crystals from Franklin and Sterling Hill are in fair agreement with the accepted formula, MnO.H₂O.

[Analyses of pyrochroite]

On the label of an original specimen of pyrochroite from the Hamburg mine, in the Brush collection, is Roepper's manuscript statement of his determination of water, H₂O, average of two analyses, 16.45 percent. The theoretical value for pyrochroite is 20.3 percent, hence his material must have been somewhat altered, or else he failed to allow for increase of weight of MnO consequent on partial oxidation.

Occurrence
At Franklin pyrochroite was found in several places on Mine Hill, being first recognized by Roepper (147) in specimens from the Hamburg mine. These now show rough rhombohedral crystals and platy aggregates, implanted on pale-pink rhodochrosite, the whole being clearly from a secondary vein in massive franklinite-zincite ore. Mr. Hancock saw some of this material when it was first taken out and reported it as transparent and of greenish color. The black manganese hydroxide noted by Brush (95) from the same mine, associated with sussexite and doubtfully referred by him to manganite, is undoubtedly to be placed under pyrochroite. As seen on the original sussexite specimens in the Brush collection, the cleavage and platy structure in the black material wore unmistakable.

The second occurrence of pyrochroite to be described was in the Buckwheat mine. All the specimens are said to have been taken from a single pocket or series of pockets that were clearly openings along a secondary vein traversing massive granular ore. These cavities are extraordinary for the great diversity of their mineral contents. Although many specimens were seen in several collections, none were available for analytical study, and the minerals could be identified only by inspection.

The walls of the cavities consist of a granular mixture of clear red manganese garnet, pink leucophoenicite, franklinite, orange-tinted zincite, and calcite. All these minerals are freely crystallized in the cavities—the garnet in simple dodecahedrons, the franklinite in octahedrons, the zincite in pale orange-yellow, sharply hemimorphic pyramids, and the calcite in beautiful transparent prisms of peculiar habit; the crystals of leucophoenicite are described elsewhere. In places the calcite completely fills the vein.

Implanted on all these are three minerals in acicular form—gageite, in white to light-brown needles of square cross section and without measurable terminations; white willemite, in excessively thin needles, in places filling the whole cavity like a mass of spun glass; and chlorophoenicite, as determined by measurement and chemical tests, in faintly yellowish to white flattened blades or needles, some of them doubly terminated. In one tiny cavity were seen minute pale-yellow scalenohedral crystals that may be smithsonite.

Later than the first-named group of minerals and coeval with the acicular growths appears the pyrochroite. It is in shining black cubelike crystals, rarely showing the base, implanted on the earlier minerals or suspended in the tangle of acicular ones. These crystals have faces too curved to yield satisfactory measurements on the goniometer, but they gave characteristic blowpipe reactions that established their nature.

Figure 51 Rhombohedral crystal of pyrochroite showing the rhombohedron and the base. Buckwheat mine, Franklin.

Tiny white rosettes of a fibrous crystalline carbonate are of the same period of growth. In some cavities a glassy, faintly pinkish mineral, in etched orthorhombic(?) crystals, coats the walls and all earlier minerals with a drusy covering or forms sparkling drops pierced by the needles of white willemite and held in. midair. This mineral could not be identified. Lastly, a second generation of calcite is rarely seen, the tiny crystals being doubly terminated and placed daintily on the tips of needles of brown gageite or on cubes of pyrochroite.

In 1909 a new locality for pyrochroite was discovered at Franklin but in what part of the mine is not known. The author is indebted to Mr. R. B. Gage for the opportunity to study the specimens as well as for the analysis given above. The crystals are in loosely aggregated crusts on a surface of matted fibers of sussexite that coats the wall of a vein in massive ore. Individual crystals do not exceed an eighth of an inch in length. On casual inspection they appear black, but close examination shows that they are really pale yellow and transparent, with a very thin coating of black oxide of manganese (the MnO₂ of the analysis). In this permanence they differ from all ether known occurrences of pyrochroite.

The crystals that yielded the new forms described above differ in habit on the same specimen in the way shown in figures 52, 53, and 54.

Figure 52 Prismatic crystal of pyrochroite showing the forms c(0001), a(1120), r(1011), s(3032), and t(9094). Franklin.
	Figure 53 Crystal of pyrochroite showing the forms c(0001), a(1120), r(1011), and a large development of t(9094). Franklin.
Figure 54 Scalenohedral crystal of pyrochroite showing the forms x(7186)? and c(0001). Probably a solution form derived from crystals of the form of figure 53. Franklin.

The second-order prism is generally dominant, some crystals being terminated by the base alone and others chiefly by rhombohedral faces variously developed. Scalenohedral crystals were also found, which are believed to be the result of solution of crystals of rhombohedral type. Their faces are rough and rounded and permitted only approximate measurement; they also show traces of a rhombohedron near the unit form. The scalenohedron, the first observed on pyrochroite, is not, then, to be regarded as a typical form but is rather to be classed as vicinal.

In 1913 pyrochroite was found in a variety of forms, especially in the new deep levels at the extreme north end of the mine. Most abundant are small seams, only a fraction of an inch thick, in massive ore, the pyrochroite being either massive granular or in small rhombohedrons on the walls of open portions of the vein, and pale brown when first taken out but soon blackening. With it are crystals of calcite and aragonite and white acicular willemite, the last two being commonly regarded by collectors as gageite.

A very abundant occurrence described by Mr. Hodgkinson was found by him near the hanging wall of the west leg of the ore body, near a mass of pegmatite and contact garnet rock. This mass was about the size and shape of a large watermelon, was surrounded by lean ore, and was associated with numerous carbonate veins as much as 5 inches thick, which trend at right angles to the layering of the ore. The specimens received from this find are composed of dull-pinkish limestone, with drusy interior surfaces of pale rhodochrosite, coated with felted masses of silky sussexite. The pyrochroite occurs in large individuals, rudely rhombohedral, with brilliant cleavage. Single sub-parallel groups of crystals are as much as l-½ inches in diameter. The mineral when found was pink or pule brown and transparent to translucent, but after some weeks it turned black, without, however, losing its brilliant luster. Minute grains of yellow and of red zincite are disseminated through the pyrochroite, the crystals of which were too rough to yield measurements.

This cavity was but a few feet from the locality of leucophoenicite described on page 105, which is entirely similar in its paragenesis. There is no doubt that both are contemporaneous with the carbonate veins, and their close association with pegmatite suggests a pneumatolytic origin. Pyrochroite is also associated with hodgkinsonite, as described on page 109.

Pyrochroite has also been found at Sterling Hill, associated with rhodochrosite and zincite in a vein in normal calcite-franklinite-willemite ore. The pyrochroite is partly altered and contains cavities, which are lined and some of them filled with crystals of the new mineral mooreite. The crystals of mooreite are covered with fluffy aggregates of fluoborite, and some of the cavities in the pyrochroite are partly filled with similar material. Further details regarding these associations are given in the descriptions of mooreite and fluoborite.