Formula: Ca6Al2(SO4)2B(OH)4(OH,O)12.26H2O
Compound sulphate, ettringite group
Specific gravity: 1.77
Hardness: 2½
Streak: White
Colour: Colourless, white

Metamorphic environments
Hydrothermal environments

Charlesite is a very rare mineral.


At the type locality, the Franklin mine, Franklin, Franklin Mining District, Sussex county, New Jersey, USA, charlesite occurs in a metamorphosed stratiform carbonate hosted zinc silicate and oxide deposit (Mindat).

Three samples from the type locality have been described in some detail. They have similar mineralogy, but indicate somewhat different parageneses.
(1) For the first sample the ore consists of franklinite and willemite with minor andradite and moderately abundant mica, likely of phlogopite - hendricksite composition. This primary assemblage is unevenly coated with a layer of grossular which is coated, in turn, with a fine-grained impure mixture of a ganophyllite-like mineral of unknown composition. Subsequent crystallisation produced a thick druse of ganophyllite in crystals up to 2.0 mm, intergrown with second-generation willemite and minor rhodonite. These three minerals are unevenly coated with a manganese-bearing chlorite and pectolite. Continued crystallisation resulted in the growth of charlesite crystals up to 6mm, which appear to have formed contemporaneously with clinohedrite. These minerals were followed by very small crystals of pectolite and xonotlite.
(2) For the second sample the assemblage consists of willemite and franklinite ore coated with a second generation of willemite crystals to 2 cm. Some willemite has been partially dissolved, leaving vugs and molds formed of granular grossular which is coated with hancockite crystals. Charlesite crystals occur in vugs among the willemite crystals and also filling cracks in fractured willemite. These willemite crystals are coated with a manganese-bearing chlorite and, in turn, by prehnite. Some interstices among these coated crystals are filled with granular clinohedrite. The entire assemblage is covered by a druse of granular datolite, followed by massive roeblingite.
(3) The third sample is the type specimen. The ore is franklinite with very minor willemite, which is encrusted with a layer of recrystallized datolite, axinite-(Mn), prehnite and hancockite. This layer is, in turn, coated with ganophyllite, followed by a layer of extremely fine-grained manganese-bearing chlorite. Subsequent crystallisation gave rise to prehnite, followed by sparse baryte and, in turn, abundant clinohedrite and charlesite. In this assemblage, the charlesite crystals in some cases are cavernous and partially dissolved.
Although these parageneses have some minerals in common, the diversity suggests some degree of spatial separation of the occurrences (AM 68.1033-1037).

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