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Formula: Cu6La(PO4)3(OH)6.3H2O
Hydrated phosphate, mixite group,
copper- and lanthanum- bearing
mineral
Crystal system: Hexagonal
Specific gravity: 3.33 calculated for the empirical formula
Hardness: 3
Colour: Yellowish green
Luminescence: Not fluorescent under UV
Solubility:
Environments
Petersite-(La) is a relatively new mineral, approved in 2017 and to date (March 2026)
reported only from the type locality.
Localities
At the type locality, the Detani River, Ohgurusu, Kiwa town, Kumano City, Mie Prefecture, Japan, the Kumano Group
consists mainly of mudstone and
sandstone, and the hydrothermal–vein type
copper deposits are widely developed along the contact area associated with
the Kumano acidic rocks. The first sample was obtained as a boulder composed of highly weathered
sandstones, and
chalcopyrite with
supergene minerals also found inside it.
Sandstones along quartz veins
have many cavities and cracks within
which supergene minerals develop. The cavities vary in size
from less than 1 mm to about 1 cm in spherical to irregular shapes. The
mixite group minerals occur in cavities coated by
chrysocolla, and the chemical compositions are different for each cavity.
These minerals all occurred as radial aggregates formed by acicular to hexagonal columnar crystals of light green to
yellowish green colour. In the petersite-(La)-dominant cavities, the
mixite group minerals coexisted with spherical
pseudomalachite and radial
malachite. Brochantite and
pyromorphite also occur in cavities, although they did not coexist with
the mixite group minerals. The
mixite group minerals are probably rare products in an oxidising and
hydrous environment involving the leaching of copper at a low temperature
during weathering.
Petersite–(La) occurs as yellowish–green acicular to hexagonal columnar crystals, up to 200 µm long and 5 µm
across. The mineral forms commonly as a radial aggregate with a silky lustre.
(Journal of Mineralogical and Petrological Sciences 115.3.286-295).
Petersite-(La) from the Detani River -
Image
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