Pyrolusite after Ramsdellite

specimen number:
2133317

location:
Mohave M&M claim, Yavapai Co., Arizona, USA

description:
The top of this matrix is covered mostly with crystals of what were once ramsdellite, these now replaced by pyrolusite. In good condition.

The Mohave M and M claim (frequently recorded as the Mohave M.M. or M&M property), located within Yavapai County, Arizona, is situated in a geologically complex region characterized by Precambrian metamorphic and igneous basement rocks. The local geology features heavily deformed Early Proterozoic schists and gneisses belonging to the Yavapai Series, which were subsequently intruded by Middle Proterozoic granitic plutons and sheared by extensive late-stage fault networks. Hydrothermal solutions moving through these structural fracture zones deposited localized lode gold, silver, and associated base-metal sulfide mineralization within steeply dipping quartz veins. Small-scale mining and surface prospecting at the claim occurred sporadically from the late 19th century through the mid-20th century, mirroring the broader operational history of the surrounding mining districts where independent operators alternated between shallow underground development and surface trenching. Active exploratory assessment and minor ore extraction peaked intermittently between 1910 and the late 1930s, fueled by regional gold rushes and favorable economic shifts during the Great Depression.

Pseudomorphs represent a fascinating mineralogical phenomenon where one mineral replaces another while perfectly preserving the external crystal form of the original parent mineral. In the specific case of pyrolusite after ramsdellite, the process is an example of a paramorph, where the chemical composition remains identical while the internal crystal structure changes. Ramsdellite, an orthorhombic manganese dioxide mineral, typically forms distinct, well-developed, prismatic or blocky crystals in low-temperature hydrothermal deposits. Because ramsdellite is chemically unstable under highly oxidizing surface conditions, it readily alters over time to pyrolusite, which is the more stable tetragonal polymorph of manganese dioxide. During this topotactic transformation, the internal atomic lattice rearranges itself without destroying the macroscopic external boundaries, resulting in a specimen that is entirely pyrolusite by composition but still displays the sharp, classic orthorhombic morphology, striations, and crystal habits of the original ramsdellite.

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