arsentsumebite after mimetite

TSNB775
Specimen
Hancock, E.P.
Exhibit 1. Arsentsumebite after mimetite; 89 mm.
Exhibit 1. Arsentsumebite after mimetite; 89 mm.
Image Credit: Malcolm Southwood
Exhibit 2. 35 mm field of view.
Exhibit 2. 35 mm field of view.
Image Credit: Malcolm Southwood
Exhibit 3. BSE image showing arsentsumebite (mid-grey) with relict inclusions of mimetite (white).
Exhibit 3. BSE image showing arsentsumebite (mid-grey) with relict inclusions of mimetite (white).
Image Credit: Martin Števko; Slovak Academy of Sciences, Bratislava, Slovakia
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Specimen Title

arsentsumebite after mimetite

Principal Mineral

Arsentsumebite

Precursor Mineral

Mimetite

Size

Cabinet; 98mm

Location in the Mine

First oxidation zone

Provenance

Hancock, E.P.

Collection

MGMH; 133414

Entry Number

Specimen; TSNB775

Description

Pseudomorphs of arsentsumebite (EDS verified; Martin Števko, pers. comm. 2024) after mimetite forming a loose intergrowth of randomly orientated slender hexagonal prisms (to 25 mm) encrusting a shard of matrix also consisting largely of arsentsumebite (?) but with a core which includes relict sulphide mineralisation.


Broken pseudomorphs show arsentsumebite forming a sheath (circa 1 mm thick) around concentrically zoned mimetite crystals (Exhibit 2). Preliminary analysis (EDS) confirms arsentsumebite as the principal replacing mineral, apparently with relatively little compositional variation, but with localised inclusions of relict mimetite (Exhibit 3).


The specimen is from the E.P Hancock Collection, acquired by Harvard in 1916, which dates it to the upper part of the first oxidation zone (i.e. 6 Level or above).