Anhydrous normal phosphate, fillowite group
Specific gravity: 3.43
Colour: Greenish yellow to yellow green, yellowish brown, reddish brown; colourless; colourless to yellow in transmitted light
Solubility: Soluble in acids
Fillowite is confined to a primary, high temperature pegmatite
assemblage (AM 66.827-842 ).
At the type locality, the Fillow Quarry, Branchville, Redding, Fairfield County, Connecticut, USA, fillowite is a primary accessory mineral in a zoned granite pegmatite containing lithiophilite (HOM) associated with triploidite, fairfieldite and reddingite (HOM, Dana).
On heating in air at 885oC fillowite slowly changes to alluaudite (AM 50.1647-1669).
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