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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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