Charlesite is a calcium aluminum sulfate boron hydroxide hydrate mineral of the ettringite group. The only extant analysis, by Jun Ito (Hurlbut and Baum, 1960) yielded: CaO 27.3, Al2O3 5.1, SiO2 3.1, SO3 12.8, B2O3 3.2, H2O 48.6, total = 100.1 wt. %.
Charlesite was described from Franklin by Dunn et al. (1983a), but is not known from Sterling Hill. The mineral had been previously considered by Hurlbut and Baum (1960) to be ettringite, to which it is structurally related.
Charlesite occurs as euhedral to subhedral crystals, tabular; the crystals have a dipyramidal habit and are up to 6 mm in diameter, although most are considerably smaller. Charlesite is colorless, but crystals may appear to be opaque white due to a surficial alteration. Charlesite is transparent with a hardness of approximately 2 1/2, a density of 1.77 g/cm3, perfect cleavage, and vitreous luster. Charlesite is weakly fluorescent with a light green or light violet color in shortwave ultraviolet; the fluorescence is weaker in longwave. Bostwick (1992) reported pale blue fluorescence in shortwave.
Identification should be made with caution; local material has not been studied with respect to the presence of ettringite. Surface alterations are composed of gypsum in part.
Charlesite occurred in the Franklin Mine, immediately above the 800 level, about 15 feet from the hanging wall and close to the north side of the Palmer Shaft pillar. A sketch map of the occurrence was given by Hurlbut and Baum (1960) and detailed descriptions of the studied specimens by Dunn et al. (1983a).
Charlesite occurs within a zone of highly recrystallized minerals, some of which are hydrated. The associated minerals are ganophyllite in tabular pseudohexagonal crystals, clinohedrite, datolite, prehnite, grossular, pennantite, willemite, pectolite, xonotlite, hancockite, roeblingite, manganaxinite, and others. Charlesite is the last mineral to form in this assemblage; colorless clear crystals, partially etched), occur within cracks in willemite. Dunn et al. (1983a) implied the existence of a number of different assemblages, but it is now clear that they are all closely related and from one occurrence. (Dunn, 1995)

 Location Found: Franklin (Type Locality)
 Year Discovered: 1981
 Formula: Ca6(Al,Si)2(SO4)2[B(OH)4](OH,O)12 · 26H2O
 Essential Elements: Aluminum, Boron, Calcium, Hydrogen, Oxygen, Sulfur
 All Elements in Formula: Aluminum, Boron, Calcium, Hydrogen, Oxygen, Silicon, Sulfur
 IMA Status: Approved 1981
Fluorescent Mineral Properties

 Shortwave UV light: Moderately bright pale blue
 Additional Information: Often coated with thin crusts of cream fluorescing gypsum
 To find out more about this mineral at minDat's website, follow this link   Charlesite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.631

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 57, No. 2 - Fall 2016, pg. 15Fluorescent Minerals of Franklin and Sterling Hill, N.J., Part 1, Richard C. Bostwick - Charlesite
View IssueV. 33, No. 2 - Fall 1992, pg. 10The Check List of Franklin-Sterling Hill Fluorescent Minerals - Charlesite (Fluorescent Info)
View IssueV. 25 No. 1 - Spring 1984, pg. 6Mineral Notes Research Reports, Charlesite
View IssueV. 24, No. 1 - Spring 1983, pg. 12Mineral Notes A Mineral New To Science, Charlesite

Charlesite crystals on datolite and hancockite from Franklin, NJ
Charlesite crystals (white) on datolite (pink) and hancockite (redish brown to brown) from Franklin, NJ. Photo by Robert A. Boymistruk.

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