Minehillite

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Formula: (K,Na)₂Ca₂₈Zn₅Al₄Si₄₀O₁₁₂(OH)₁₆
Phyllosilicate (sheet silicate), reyerite group
Crystal System: Hexagonal
Specific gravity: 2.93 measured, 2.94 calculated
Hardness: 4
Streak: White
Colour: Colourless, white
Luminescence: Fluoresces medium dull violet in short wave and duller violet in long wave UV
Common impurities: Fe,Mn,Mg,H₂O
Environments

Metamorphic environments
Hydrothermal environments

Although minehillite was approved in 1984, to date (August 2022) it has been reported only from the type locality.

Localities

At the type locality, the Franklin Mine, Franklin, Franklin Mining District, Sussex County, New Jersey, USA, minehillite occurred as a secondary low-temperature hydrothermal mineral formed by replacement of associated minerals in the metamorphosed stratiform zinc deposit. The minehillite specimens studied were all from mineral collections; the Franklin Mine was closed in 1954. Minehillite appears to have been moderately abundant since several dozen specimens are now known. It occurred in varied parageneses, which suggests that minehillite was not restricted to a single occurrence but was distributed over some region of the deposit.
The holotype minehillite consists of 5 mm plates of colourless material, which appears white in the aggregate. These plates form a layer up to 1 cm thick and 5 cm wide encrusting grey microcline with allanite crystals liberally distributed throughout; the minehillite appears to have formed on a broad, flat fracture in microcline.
Other specimens were found to be closely related. On one of these, minehillite forms at the contact between feldspar and a fine-grained mixture of wollastonite, grossular and vesuvianite. In this occurrence, minehillite occurs as distorted aggregates a few mm in diameter, with abundant inclusions of native lead, giving the minehillite a noticeable and characteristic greyish-black colour.
In another assemblage, minehillite forms tight, distorted aggregates in a band 1 cm thick between broad-bladed margarosanite and pink calcite that contains crystals of clinopyroxene; this minehillite is replacing all the other phases.
All of the elements in minehillite can be derived from the precursor minerals with the exception of zinc. It may be that zinc was present as a now-replaced phase such as willemite or zincite, or, more likely, that zinc was introduced in solution.
The occurrence of native lead included within minehillite suggests that components for formation of minehillite can originate from the breakdown of margarosanite (AM 69.1150-1155).

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