Sorosilicate (Si2O7 groups), epidote group, strontium- and manganese- bearing mineral
Specific gravity: 3.84 calculated
Hardness: 6 to 7
Streak: Brownish red
Colour: Deep red
Common impurities: Fe
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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