Structure refinements of coquimbite and paracoquimbite from the Hongshan Cu-Au deposit, NW China

Autor(en)
Zhuming Yang, Gerald Giester
Abstrakt

The crystal structures of coquimbite and paracoquimbite from the Hongshan Cu–Au deposit, NW China, were refined by single-crystal X-ray diffraction, including the positions of all non-H atoms and hydrogen atoms. The unit-cell parameters of coquimbite are a = 10.9644(15), c = 17.090(3) A, and V = 1769.6(5) A

3, Z = 4 with space group P31c. The crystal structure was refined based on 1352 unique reflections to R1(F) = 0.0301, yielding the formula Fe

1.64Al

0.36(SO

4)

3·9H

2O. Among the three cation positions of coquimbite, the isolated octahedral one is dominantly occupied by Al, i.e. Al

0.660(3)Fe

0.340(3). Consequently, the average Al–O interatomic distance is 1.9155 A, smaller than the Fe–O distances, 2.002 A for Fe(1), and 1.989 A for Fe(2). Based on the structure data available for coquimbite in the literature, the ideal formula should be expressed as AlFe

3(SO

4)

6·18H

2O (Z = 2) or Fe

2

xAl

x(SO

4)

3·9H

2O, x ∼ 0.5 rather than Fe

3

+

2(SO

4)

3·9H

2O. The simplified formula for coquimbite from Hongshan is (Al

0.66Fe

0.34)Fe

3(SO

4)

6·18H

2O. The unit-cell parameters of paracoquimbite are a = 10.9631(16), c = 51.473(10) A, and V = 5357.7(15) A

3, Z = 12 with space group R3. The crystal structure was refined using 2733 unique reflections to R1(F) = 0.0550, which led to the formula Fe

1.91Al

0.09(SO

4)

3·9H

2O. There are five non-equivalent octahedrally coordinated metal sites for Fe in the structure of paracoquimbite. The range of Fe

[6]–O interatomic distances in paracoquimbite (1.966 A–2.016 A) can well be compared with those found in various sulfate minerals of ferric iron. Both minerals are characterized by a complex system of hydrogen bonds involving three types of H

2O molecules, respectively: those of [Fe

3(SO

4)

6(H

2O)

6]

3 clusters, of isolated [Fe(H

2O)

6]

3

þ or [Al(H

2O)

6]

3

þ octahedra and further interstitial (H

2O) groups. The interstitial H

2O molecules resemble a cyclohexane-like chair and are held in the structure solely by hydrogen bonding. In coquimbite H

2O

W(2) is disordered with two alternative orientations (H2B1 and H2B2), in paracoquimbite interstitial H

2O consists of two H

2O molecules, H

2O

W(3) and H

2O

W(4). The range of the O–H stretching frequencies between ∼2750 and ∼3650 cm

1 from infrared spectroscopy for paracoquimbite is in fair accordance with the calculated values between ∼2874 and ∼3600 cm

1 from both the d(O…O) lengths and the d(H…O) lengths.

Organisation(en)
Institut für Mineralogie und Kristallographie
Externe Organisation(en)
Chinese Academy of Sciences (CAS)
Journal
European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences
Band
30
Seiten
849-858
Anzahl der Seiten
10
ISSN
0935-1221
DOI
https://doi.org/10.1127/ejm/2018/0030-2752
Publikationsdatum
10-2018
Peer-reviewed
Ja
ÖFOS 2012
105116 Mineralogie, 105113 Kristallographie
Schlagwörter
ASJC Scopus Sachgebiete
Geochemistry and Petrology
Link zum Portal
https://ucrisportal.univie.ac.at/de/publications/6b474c99-fa37-42d6-96c9-7667a2a3b37c