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Acanthite


Acanthite is silver sulfide, a simple compound with complex thermal behavior. Silver sulfide is cubic at high temperature (Emmons et al., 1926) and monoclinic at room temperature (Ramsdell, 1943; Frueh, 1958). When describing curious spiked crystals of silver sulfide from Germany, Kenngott (1855) named acanthite using Greek acantha for thorn. For morphologically cubic silver sulfide, Haidinger (1845) proposed mineral name argentite. The advent of X-ray diffraction in 1909 permitted determination of internal atomic arrangements in crystals, i.e. crystal structures. Generally in minerals, the outward crystal form reflects the symmetry of the crystal structure, but not in argentite, which gave the same pattern as that of acanthite. Outwardly cubic argentite is internally monoclinic, or simply "acanthite paramorphs after argentite" (Ramsdell, 1927). The name argentite now applies to the high temperature cubic structure, which is unquenchable at room temperature and consequently not an approved mineral name. Sadovnikov & Gusev (2019) report a transition temperature of 449-450 K (177-178°C). Cubic crystals of acanthite form above the transition temperature, making them de facto geo-thermometers.

This is a dendritic group of acanthite, largely globular in form. The rotund globules to 1.4 × 1.4 × 1.0 cm show vestigal cubic form. It is unclear if this acanthite became globular by melting or by dissolution; such is the ongoing work of economic geology. Nevertheless this is an intriguing specimen of bright metallic acanthite without matrix. A bit of drusy quartz is the only associated mineral.

Price: $2000

Item code: MS1688

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