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Order-disorder twinning of malachite

Emil Makovicky and Stefano Merlino
Order-disorder twinning of malachite
The Canadian Mineralogist (July 2019) 57 (4): 475-488


Malachite has a layered structure composed of triangular CO (sub 3) groups and CuO (sub 2) (OH) (sub 2) coordination squares of two types. In 2D projection, each (201) layer can be decomposed into two OD strips: the A strip, containing CO (sub 3) groups and square co-ordinations of Cu2, with idealized symmetry pgm2, and the B strip, without CO (sub 3) groups, with symmetry p2 (the b axis of malachite is the infinite strip direction). In the 3D structure, these strips form the 'A' OD-layers with symmetry reduced by coordination requirements to approximately p22 (sub 1) 2, which alternate with the 'B' OD-layers with layer symmetry p1. The B-layer can assume two positions, which are related by a 180 degrees rotation axis running through the C1 and O1 atoms of the CO (sub 3) group in the A-layer. This axis is oriented close to [100]*. The fit of the A-layer and of the coplanar rotated B-layer is hindered by the presence of important partial gaps and overlaps at the boundary. Inclining these two structure portions towards one another, together with small positional adjustments, appears to alleviate the misfit problems; the two layer portions so related form an angle of about 124 degrees , i.e., they form a twin relationship. This desymmetrization of the malachite structure away from the ideal model results in occasional twinning instead of the fully developed monoclinic polytype structure which is derived in this paper. In the structure of the Cu-Zn analogue, rosasite, the above-described match problem is solved in a different way, by shifting a Zn coordination octahedron (which replaces Cu2 in malachite) into the interlayer space. Malachite polytypes 1 and 2 and rosasite illustrate three different orientations of P2 (sub 1) /a symmetry elements with respect to the structural layers, resulting in three different structure configurations.

ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 57
Serial Issue: 4
Title: Order-disorder twinning of malachite
Affiliation: University of Copenhagen, Institute of Geoscience and Natural Resources Management, Copenhagen, Denmark
Pages: 475-488
Published: 20190715
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 30
Accession Number: 2020-006648
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
S13°00'00" - N05°15'00", E12°00'00" - E32°00'00"
Secondary Affiliation: Universita di Pisa, ITA, Italy
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202006
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