Habit
The crystals of larsenite are slender needles 10 to 20 times as long as they are thick, forming an interlacing network in vein cavities. Many of them reach from wall to wall and show no terminations, but a few stand with one end free and can be detached for measurement. Rarely the mineral has the form of thin lustrous plates.

The prism zone is generally rounded by striation, only one measured crystal showing a pronounced flattening parallel to the brachypinacoid. The prism s(120), to which the cleavage is parallel, was seen on all the crystals and gives good reflections, whereas in m(110), n(210), and y(150) are only narrow linear faces.

The termination is generally dominated by the pyramid A(142), whose faces are excellent and give consistent readings.

Figure 112 shows the prevailing habit; in figure 113 the brachydome is added; figure 114 shows it habit seen on but one crystal; and the platy habit is illustrated in figure 115.

Figure 112 Prismatic crystal of larsenite showing the usual termination, with the forms b(010), m(110), s(120), A(142), and B(9.8.18). Franklin.
	Figure 113 Plan of a prismatic crystal of larsenite showing the forms of figure 112 and also w(012). Franklin.
Figure 114 Plan of a prismatic crystal of larsenite showing the forms m(110), s(120), q(122), t(132), and A(142). Franklin.
	Figure 115 Plan of a crystal of larsenite of tabular habit, showing the forms b(010), s(120), and A(142). Franklin.

Forms not shown in the figures are subordinate and are generally seen only as linear faces. The needles are very slender, so that the terminal faces on all crystals are exceedingly minute. Because of the high luster of the mineral, however, they give good measurements despite their small size.

Composition
The following analysis shows the composition of larsenite:

The material analyzed was known to contain clinohedrite, and the lime and water found were regarded as due to that mineral. After deducting 9.26 percent of clinohedrite, the remainder gave a ratio fairly close to that required by the formula PbZnSiO₄, which is therefore adopted as the formula of larsenite.

Occurrence
Larsenite was first found in 1928 and, with the closely related calcium larsenite, was described in papers by Palache, Bauer, and Berman (256, 259). The first specimens were found on the picking table at Franklin, and the mineral was afterward located in the mine at the north end, 20 feet above the 400-foot level in top slice 1080. Part of the specimens are in veins cutting ore, and part have replaced rather coarse, massive willemite-franklinite ore. The veins are sharply bounded against the granular ore and differ in their contents from place to place.

The earliest mineral to form was a massive pink garnet, identical in appearance and refractive index (1.885) with garnet from another specimen, shown by partial analysis to be andradite. Upon this is a coating of hodgkinsonite of the same pink color as the massive garnet. Most surfaces of the open veins show rather indistinct crystals, with rounded faces, of pale-pink to almost colorless hodgkinsonite. In several specimens devoid of lead silicates the only minerals later than hodgkinsonite are calcite in slender needles or clear prismatic crystals, botryoidal coatings of smithsonite, granular orange-colored zincite, and a last coating of snow-white needles of willemite forming delicate rosettes. In other parts of the open veins hodgkinsonite was followed by an abundant layer of crystalline clinohedrite with which the slender needles and plates of larsenite are contemporaneous. A few spots of dull white massive roeblingite covering clinohedrite crystals, were found in one of the specimens. Larsenite is far less abundant than calcium larsenite, and in most specimens the two are not associated, but in two places crystals of larsenite were found in cavities with calcium larsenite. Furthermore, larsenite is found only in the vein material and not in the replaced ore.

It is worthy of note, in view of the discovery of these lead silicates, that in two analyses of glaucochroite (see page 79) some lead is reported, indicating the possible presence of the larsenite molecule.