iddingsite

Iddingsite is a hydrous alteration product of olivine, typically forming when olivine-bearing rocks interact with liquid water at low temperatures. It is not a single mineral but an assemblage that can include hydrated iron oxide and hydroxide minerals, such as goethite and hematite, together with clay minerals and minor silica. The exact composition and texture of iddingsite vary with the conditions of alteration, often appearing as rims, patches, or patchy regions around olivine grains.

Formation and occurrence: In terrestrial rocks, iddingsite rims commonly develop on olivine in ultramafic rocks exposed to weathering, such as peridotites and basalts, under near-surface conditions. In meteorites, especially olivine-rich chondrites, iddingsite forms during aqueous alteration on the parent body, recording past water-rock interaction. The mineral is thus used as an indicator of past aqueous activity and its extent.

Identification and significance: Iddingsite is identified through petrographic microscopy and electron microscopy, with supporting analyses from X-ray diffraction, Raman spectroscopy, and energy-dispersive spectroscopy to detect iron oxide/hydroxide phases and hydrated minerals. Its presence signals that olivine experienced liquid water and oxidation conditions, providing constraints on the alteration environment, temperature, and history of the rock or meteorite.

Name and history: The alteration product is named after the American petrologist Earl J. Iddings, who contributed to early studies of olivine weathering and mineral assemblages in igneous and metamorphic contexts.