Englishite Locality: Tip Top Mine (Tip Top pegmatite), Fourmile , Custer District, Custer County, South Dakota , USA ( Locality at mindat.org ) Pearly Englishite balls together with some Montgomeryite sprays. Field of view 5 mm. Specimen and photo Leon Hupperichs. — Photo: Leon Hupperichs · CC BY-SA 3.0
Englishite
Englishite is an IMA-recognized mineral species in the phosphate class, with the chemical formula K₃Na₂Ca₁₀Al₁₅(OH)₇(PO₄)₂₁·26H₂O.
It crystallizes in the monoclinic crystal system (space group A2/a).
Key properties: Mohs hardness 3; specific gravity 2.65–2.68; Vitreous, pearly on cleavages luster; optically biaxial.
IMA symbol Egs, grandfathered status — known before the IMA’s founding, named in 1930.
Found in variscite nodules (Little Green Monster mine, Utah, USA); in a complex granite pegmatite (Tip Top mine, South Dakota, USA).
It is found associated with Variscite, montgomeryite, wardite, millisite, crandallite (Little Green Monster mine, Utah, USA); montgomeryite, eosphorite–childrenite, tiptopite, roscherite.
The mineral was named to honor George Letchworth English (1864–1944), New York City, New York, USA, American mineral dealer and collector.