Habit
Hodgkinsonite is found both granular and in crystals which are generally small and of acute pyramidal habit determined by the development of the unit prism and the steep hemipyramid r(221).

Figure 166 Acute pyramidal crystal of hodgkinsonite formed by the combination of the forms m(110), r(221),and s(011). Franklin.

The other faces are generally seen in modifications of the acute forms, but in one lot of specimens the crystals are of stout prismatic habit. On etched crystals the pyramid r(221) is replaced by a group of facets whose average position corresponds to the form q(552). The clinodome and prism faces are smooth, but generally dull and poorly reflecting. The faces of other forms, although small, give good reflections for measurement.

The crystallographic data in the first description of hodgkinsonite by Palache and Schaller (203) were based on poor and scanty material. Much better crystals were later obtained, and the results here presented include both the published data and those obtained in later studies. The measured crystals were generally minute, rarely more than 0.1 inch long, but the faces gave good reflections. The illustrations show the considerable range of habit.

Figure 167 Plan of a crystal of hodgkinsonite showing the forms c(001), m(110), s(011), o(021), v(403), w(201), t(401), p(111), r(221), and n(311). Franklin.
	Figure 168 Projection on the clinopinacoid of a crystal of hodgkinsonite showing the c(001), m(110), s(011), o(021), w(201), t(401), r(221), P(111), E(421), and A(261). Franklin.
Figure 169 Plan of a crystal of hodgkinsonite showing the form c(001), m(110), l(210), s(011), o(021), v(403), w(201), t(401), r(221), n(311), P(111), E(421), z(625), y(312), and B(532). Franklin.

The stout prismatic crystals of figures 170 and 171 were in cavities with crystals of hetaerolite and willemite.

Figure 170 Stout prismatic crystal of hodgkinsonite showing the forms c(001), l(210), and s(011). Franklin.
	Figure 171 Stout prismatic crystal of hodgkinsonite similar to that shown in figure 170 but showing also the form m(110). Franklin.

They were clear yellow and because of their pronounced pseudo-orthorhombic habit they were not recognized as until they were measured.

In 1923 Gordon (227) described small crystals of hodgkinsonite with forms not observed by the author. Two of them—d(176) and e(1.12.6)—approximate closely the position of clinodomes but are regarded by the author as definitely established pyramids.

In 1928 Palache (257) described hodgkinsonite crystals of new and complex habits which were measured and drawn by L. W. Lewis, The crystals of combinations 16 and 17 are remarkable in having the clinodome zone dominant, the prisms being merely minute truncations. Combinations 18 and 19 were the most complex crystals of this mineral yet found. Figures 174, 175, and 176 show two of several small gemlike doubly terminated pink crystals delicately attached to larger, singly terminated crystals of the same habit.

Figure 174 Plan, drawn with the orthoaxis toward the front instead of in the usual position, of a crystal of hodgkinsonite showing the forms c(001), m(110), l(210), s(011), o(021), v(403), w(201), t(401), R(401), r(221), p(111), f(211), g(421), F(223), and E(421). Franklin.
	Figure 175 Clinographic projection, also drawn with the orthoaxis toward the front, of the crystal whose plan is shown in figure 174.
Figure 176 Plan of a complex crystal of hodgkinsonite showing the forms c(001), m(110), l(210), s(011), o(021), v(403), w(201), t(401), R(401), p(111), G(223), r(221), i(441), h(623), f(211), g(421), k(243), H(131), E(421), and M(621). Franklin.

One end of a doubly terminated crystal only 2 millimeters in its greatest diameter, shown in figure 176, gave reflections from 44 faces of 26 forms, 15 of which belong to forms new to the mineral. The crystals are elongated parallel to the vertical axis, with the unit prism and the pyramid r(221) dominant and in approximately equal development.

Physical character
The cleavage is very perfect parallel to the base; the specific gravity, determined with the pycnometer on a gram of selected fragments, is 3.91 ; and the hardness is slightly less than 5. The color is typically clear bright pink but ranges to clear reddish brown and rarely to distinct yellow.

Optical character
Hodgkinsonite is optically biaxial and negative; 2V is 50° to 60° ; Y = b ; Z /\ c = 38° ; the axial plane is parallel to the clinopinacoid. a = 1.724 (lavender), b = 1.742 (almost colorless), g = 1.746 (lavender) (Berman).

Pyrognostics
Hodgkinsonite decrepitates when held in the blowpipe flame but fuses readily and quietly to a brown enamel. Heated in the closed tube it decrepitates strongly, splitting into numerous thin cleavage flakes that yield water and turn brown. It is readily soluble in acids, yielding gelatinous silica.

Composition
Hodgkinsonite is a basic zinc-manganese silicate, which generally contains practically negligible amounts of iron, calcium, and magnesium. Schaller (203) found a doubtful trace of lead but no iron or chlorine.

Except for unimportant differences in the amounts of the minor constituents, the analyses agree closely and give a molecular ratio of ZnO : MnO : SiO₂ : H₂O as 2 : 1 : 1 : 1 , from which the formula Mn(ZnOH)₂SiO₄ is derived.

[Analyses of hodgkinsonite]

Occurrence
Hodgkinsonite was discovered in Franklin in 1913 by H. H. Hodgkinson, for whom it is named. It was found in the northern part of the ore body, in that part of the Parker mine formerly known as the Hamburg mine, near the hanging wall of the ore body, between the 850-foot and 900-foot levels. It has since been found in several specimens and may be considered an abundant mineral

It forms seams in massive granular ore of the typical willemite-franklinite mixture, the seams being generally very thin with but a film of the mineral, which is almost everywhere associated with barite and not uncommonly with native copper. In places the seams thicken to thin veins, and some crystals of the mineral are as much as 0.8 inch across, sharply angular, and apparently with good crystal faces, but in reality determined in their form almost wholly by the older, platy barite that encloses them. The clear-pink color and brilliant cleavage of the hodgkinsonite, together with the snow-white barite, make such specimens both striking and attractive in appearance. One mass of ore with a surface 8 inches square is at least half covered with hodgkinsonite.

Only one vein has been found in which the angular cells formed by the intersecting plates of barite were not wholly filled by hodgkinsonite, so that the latter mineral was free to develop crystal faces. Other crystals were found in cavities in thicker veins free from barite. These crystals, the largest half an inch long, were much affected by solution, the faces being generally dull or faceted. They were accompanied by black rhombohedral crystals of pyrochroite and scalenohedral crystals of calcite, both later than and encrusting the hodgkinsonite, which is implanted directly on willemite and franklinite and in one specimen on manganese garnet.

In an open vein found in 1927 very fine crystals of hodgkinsonite were found with well-developed crystals of green willemite, tephroite, and barite. The hodgkinsonite crystals were implanted on the vein walls and were of the habit shown in figure 172, with the clinodome zone dominant.

Figure 172 Crystal of hodgkinsonite showing the c(001), m(110), l(210), s(011), o(021), v(403), w(201), t(401), r(221), E(421), p(111), and P(111). Franklin. A, Plan; B, clinographic projection.

Some of them were of a rhodonite-pink color and some of a clear-red tone like that of some spessartite. Veins filled solidly with hodgkinsonite yielding cleavage surfaces an inch across were found in the Stanton collection. The mineral is common in veins throughout the northern part of the mine at Franklin.

Figure 173 Projection on the clinopinacoid of a crystal of hodgkinsonite showing the forms c(001), m(110), s(011), p(111), r(221), P(111), and E(421). Franklin.