Formula: Pb2(UO2)(AsO4)2.nH2O
Hydrated normal arsenate, forms a series with parsonsite, and forms oriented intergrowths with hügelite (Dana)
Specific gravity: 6.445 calculated
Hardness: 2½ to 3
Streak: Light yellow
Colour: Yellow

Sedimentary environments
Hydrothermal environments

Hallimondite is a rare mineral that was first discovered on a museum specimen from an oxidising arsenic-lead bearing deposit, formed by alteration of galena (HOM).


At the type locality, the Michael Mine, Weiler, Seelbach, Seelbach, Ortenaukreis, Freiburg Region, Baden-Württemberg, Germany, hallimondite occurs as finely crystalline yellow material in druses of chert breccia, associated with hügelite and mimetite, rarely as crystals up to 0.3 to 0.4 mm on hügelite. The hallimondite occurs in the oxidation zone of an arsenic-lead bearing deposit containing various sulphides and much native arsenic, along with zeunerite, nováčekite, metaheinrichite, kasolite, mimetite, adamite, hügelite, widenmannite, weilerite, baryte, galena and quartz (AM 47.414-420, HOM).
Hallimondite is found as small crystals and fine-grained coatings in cavities and fractures of quartz. It contains inclusions of baryte and galena. Earthy mimetite in places coats hallimondite aggregates, thus proving that hallimondite is older. On the other hand hallimondite evidently was formed later than hügelite, for in one specimen crystals of hallimondite were found as an incrustation on hügelite. The formation of hallimondite and hügelite is due to the action of arseniferous and uraniferous waters on galena. The arsenic acid has been derived by oxidation of native arsenic which is locally abundant. No primary uranium minerals occur in the vein, so it is likely that the uranium was derived from the wall rock (AM 50.1143-1157).

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