Leningradite is the product of fumarole activity in the Great Tolbachik Fissure Eruption. It occurs in relatively small amounts in fumarole incrustations, on the southwestern crest of the second slag cone of the Northern Breakthrough of the eruption, above the vent, where it was deposited in the upper parts of the vugs. The mineral was found both as single poorly faceted red-brown rhomboidal crystals or flakes, or as intergrowths of fine tabular crystals. The crystals and flakes measured a maximum of 0.3 mm across (ordinarily 0.1 mm) in the plane of flattening and were about 0.3 mm thick. The intergrowth aggregates had the appearance of microscopic spheres or globules to 0.6 mm in diameter (usually 0.2 to 0.3 mm), with rough maroon-brown surfaces. Internally, some of the spherical particles resembled spherulites, with the crystals oriented in an orderly fashion from the center to the edges in other cases the crystals were in a spiral arrangement