Minehillite



Minehillite is a potassium calcium zinc aluminum silicate hydroxide mineral and is closely related to reyerite (Dunn et al., 1984a). Minehillite was first described from Franklin by Dunn et al. (1984a). It has not been found at Sterling Hill.
Minehillite occurs as massive aggregates of mostly warped, lamellar crystals, up to 1.0 cm in size, and twinned by a 60° rotation about c. It is colorless, but may appear white in the aggregate or locally black due to inclusions of native lead. The luster is vitreous, but pearly on the perfect cleavage. The density is 2.93 g/cm3. Minehillite fluoresces medium dull-violet in shortwave ultraviolet and duller violet in longwave. It is easily distinguished from margarosanite by its indices of refraction and by the intense fluorescence of the latter; prehnite is likewise distinguished by its fluorescence.
Minehillite was found in the northern end of the west limb of the Franklin Mine in an area near the Palmer Shaft pillar, where Pb-silicates were discovered (Dunn, 1985b). It is associated with margarosanite, wollastonite, allanite, microcline, diopside, grossular, vesuvianite, and calcite. It replaces most of these minerals and is likely a secondary, low-temperature, hydrothermal, replacement mineral. It forms as reaction rims and occurs as aggregates on microcline.
Minehillite does not accept Pb when margarosanite is replaced; platelets of native lead are present in many specimens as inclusions. They form gray to black rims indicating the extent of minehillite replacement of the pre-existing assemblage. The presence of Pb, however, does not imply that conditions for the formation of this assemblage were more strongly reducing than the overall conditions at Franklin.
The presumption of Dunn et al. (1984a) that there were numerous host assemblages is apparently in error; subsequent study indicates there was likely but one very varied assemblage. Minehillite specimens are moderately common; much of the material was preserved by collectors and miners by serendipity because of the strong fluorescence of the associated wollastonite and margarosanite. (Dunn, 1995)


 Location Found: Franklin (Type Locality), unique to Franklin/Ogdensburg area
     
 
 Year Discovered: 1984
     
 Formula: (K,Na)2-3Ca28Zn4Al4Si40O112(OH)16
 Essential Elements: Aluminum, Calcium, Hydrogen, Oxygen, Potassium, Silicon, Zinc
 All Elements in Formula: Aluminum, Calcium, Hydrogen, Oxygen, Potassium, Silicon, Sodium, Zinc
     
 IMA Status: Approved
     
Fluorescent Mineral Properties

 Shortwave UV light: Weak violet
 Mid wave UV light: Violet-blue, moderate brightness
 Longwave UV light: Weak pale yellow
     
 To find out more about this mineral at minDat's website, follow this link   Minehillite

     
 References:
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.501


The Picking Table References
 PT Issue and PageDescription / Comment
V. 58, No. 1 - Spring 2017, pg. 13Fluorescent Minerals of Franklin and Sterling Hill, N.J., Part 2, Richard C. Bostwick - Minehillite
View IssueV. 45, No. 1 - Spring 2004, pg. 12The Art of Fluorescent Mineral Photography, With Special Attention to the Minerals of Franklin and Sterling Hill Photographing the More Popular Franklin and Sterling Hill Fluorescent Minerals - Minehillite
View IssueV. 41, No. 1 - Spring 2000, pg. 10Minehillite (article mentions close visual appearance of margarosanite and minehillite)
View IssueV. 33, No. 2 - Fall 1992, pg. 11The Check List of Franklin-Sterling Hill Fluorescent Minerals - Minehillite (Fluorescent Info)
View IssueV. 26, Combined Issue 1985, pg. 8Mineral Notes New To Science, Minehillite
View IssueV. 25 No. 1 - Spring 1984, pg. 5Mineral Notes New To Science, Minehillite
     
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