page_title: Silver iodide
section_title: Structure
hierarchical_section_title: Silver iodide / Structure
caption_attribution_description: English: Iodargyrite Locality: Broken Hill, Yancowinna County, New South Wales, Australia (Locality at mindat.org) Size: 9.3 x 6.3 x 5.7 cm. Iodargyrite is one of the relatively rare silver iodine halides for which the renowned Broken Hill deposit is justifiably famous. Waxy, translucent, olive-green to yellow-green, iodargyrite crystals in clusters or as isolated crystals richly and attractively cover the sculptural, vuggy, gossan matrix. This is overall a very rich, large, and unusually attractive specimen. Classic, older, combination material from this world-class locality - it probably dates to the early 1900s to 1940s era here. Deutsch: Jodargyrit Fundort: Broken Hill, Yancowinna County, New South Wales, Australien (Fundort bei mindat.org) Größe: 9.3 x 6.3 x 5.7 cm.
context_page_description: Silver iodide is an inorganic compound with the formula AgI. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a gray coloration. The silver contamination arises because AgI is highly photosensitive. This property is exploited in silver-based photography. Silver iodide is also used as an antiseptic and in cloud seeding.
context_section_description: The structure adopted by silver iodide is temperature dependent:
  Below 420 K, the β phase of AgI, with the wurtzite structure, is most stable. This phase is encountered in nature as the mineral iodargyrite.
  Above 420 K, the α phase becomes more stable. This motif is a body-centered cubic structure which has the silver centers distributed randomly between 6 octahedral, 12 tetrahedral and 24 trigonal sites.  At this temperature, Ag⁺ ions can move rapidly through the solid, allowing fast ion conduction. The transition between the β and α forms represents the melting of the silver (cation) sublattice. The entropy of fusion for α-AgI is approximately half that for sodium chloride (a typical ionic solid). This can be rationalized by considering the AgI crystalline lattice to have already "partly melted" in the transition between α and β polymorphs.
  A metastable γ phase also exists below 420 K with the zinc blende structure.
caption: The golden-yellow crystals on this mineral sample are iodargyrite, a naturally occurring form of β-AgI.
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