Abstract
The pearceite-polybasite group of minerals, general formula
[M
6
T
2
S
7
][Ag
9
CuS
4
]
with M = Ag, Cu; and T = As, Sb, show a crystal structure which can be described
as the succession, along the c axis, of two pseudo-layer modules: a
[M
6
T
2
S
7
]
2–
A
module layer and a
[Ag
9
CuS
4
]
2+
B
module layer. Copper is present in one structural position of the
B
module layer and replaces Ag in the only fully
occupied M position of the
A
module layer. When the Cu
content is >4.00 a.p.f.u., the structural position of the
A
module layer becomes Cu-dominant and,
consequently, the mineral deserves its own name. In this paper we report the
crystal-chemical characterization of two Cu-rich members exhibiting the 111
unitcell type (corresponding to the Tac polytype). One sample (space group
(
P
)
m
1,
a
7.3218(8),
c
11.8877(13) Å,
V
551.90(10) Å
3
, Z
= 1) having As >Sb and with the structural position of the
A
module layer dominated by Cu, has been named
cupropearceite and the other sample (space group (P3̄)m1, a 7.3277(3), c
11.7752(6) Å,
V
547.56(8) Å
3
, Z =
1) having Sb >As has been named cupropolybasite.
Both the new minerals and mineral names have been approved by the
IMA-CNMNC.
