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Formula: CaSr(Mn3+2Al)[Si2O7][SiO4]O(OH)
Sorosilicate (Si2O7 groups), epidote group,
strontium- and manganese- bearing
mineral
Crystal System: Monoclinic
Specific gravity: 3.84 calculated
Hardness: 6 to 7
Streak: Brownish red
Colour: Deep red
Common impurities: Fe
Environments
Metamorphic environments
Hydrothermal environments
Localities
At the Sanbagawa metamorphic belt, Japan, tweddillite was found together with
braunite and hollandite in a
piemontite quartzose
schist. It is estimated that these samples formed at a temperature of
480 to 580oC and 0.7 to 1.0 GPa pressure
(MM 66.137-150).
At the type locality, the Wessels Mine, Hotazel, Kalahari manganese field, Northern Cape, South Africa,
tweddillite occurs in calc-silicate rocks formed by
hydrothermal alteration of primary, sedimentary
manganese ores. The tweddillite crystals form dark-red ‘suns’ of very
thin (~0.02 mm) radiating blades on the hanging wall of the ore-body. Associated minerals include
serandite, pectolite and
braunite
(MM 66.137-150).
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