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Formula: BaCa2Mn3+4(Si3O10)(Si2O7)(OH)4.3H2O
Unclassified silicate, barium- and
manganese- bearing mineral
Crystal System: Orthorhombic
Specific gravity: 3.525 calculated
Hardness: 3½
Streak: Beige
Colour: Orange-brown to red-brown
Luminescence: Not fluorescent under UV
Solubility: Does not react with hydrochloric acid at room temperature
Environments
Sedimentary environments
Metamorphic environments
Demagistrisite is a relatively new mineral, approved in 2018 and to date (February 2024) reported only from
the type locality.
Localities
At the type locality, the Cerchiara Mine, Borghetto di Vara, La Spezia Province, Liguria, Italy, the ore, which
consists of rhythmic interlaying of braunite-bearing metasediments, 5 to
15 cm thick, and hematite-rich
chert, was produced by precipitation from submarine hydrothermal vents
and reworked by turbiditic resedimentation; subsequently, the sedimentary-diagenetic deposits were re-equilibrated
under prehnite-pumpellyite facies
conditions leading to a braunite +
quartz stable assemblage. Demagistrisite occurs in association with
cerchiaraite-(Mn),
namansilite,
noelbensonite, orientite,
richterite, ruizite and
saponite in matrix consisting of
braunite, calcite,
cryptomelane, orthoclase
and quartz.
Demagistrisite crystals occur as tightly intergrown blades or as millimeter-sized prisms and needles with
square cross-section, typically with irregular terminations, and rarely terminated by a low-angle pyramid. The
mineral is translucent to transparent, orange brown to red brown, with a beige streak and a vitreous lustre
(CM 59.1.91–105).
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