Quick
Search: 		  
advanced search
 GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help 
American Mineralogist GSW 2008 Users' Group Meeting
JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
American Mineralogist; April 2008; v. 93; no. 4; p. 501-507; DOI: 10.2138/am.2008.2661
© 2008 Mineralogical Society of America
This Article
Right arrow Figures Only
Right arrow Full Text
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Google Scholar
Right arrow Articles by Kyono, A.
GeoRef
Right arrow GeoRef Citation

Compositional variability and crystal structural features of guanacoite

Atsushi Kyono*

Division of Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan

Correspondence: * E-mail: kyono{at}geol.tsukuba.ac.jp

Guanacoite with high Mg content from the type locality (El Guanaco mine, Chile) was investigated to elucidate the crystal-chemical characteristics of this arsenate mineral. In this study, WDS X-ray intensity maps revealed that guanacoite single crystals from the investigated specimen show distinct and characteristic compositional zoning, which varies cyclically along [100], which is in the direction of the elongation of the prismatic crystals (Witzke et al. 2006); the boundaries separating different compositional regions lie perpendicular to the prism faces. The crystal structure of a selected fragment [a = 5.459(2), b = 16.808(9), c = 6.917(3) Å, β = 100.44(1)°, V = 624.1(5) Å3, P21/c, and Z = 2], was solved using direct methods and refined to an R index of 3.09% for 1385 observed [Io > 4{sigma}(Io)] reflections measured at 103 K using MoK{alpha} X-radiation. The chemical formula based on the refinement is (Cu0.85Mg0.15)2Mg2(Mg0.83Cu0.17)(OH)4(H2O)4(AsO4)2. The structure model obtained is in good agreement with the previous structure determination, although the occupancies of the three M sites (M = Mg or Cu) are distinctly different, i.e., the sites are considerably richer in Mg, especially M1 and M3. These structural results suggest that guanacoite’s chemical formula should be enlarged from the Cu-rich composition Cu2Mg2(Mg,Cu)(OH)4(H2O)4(AsO4)2 to (Cu,Mg)2Mg2(Mg,Cu)(OH)4(H2O)4(AsO4)2 to include the Mg-rich members (Mg up to 3.395 apfu). It appears that the chemical variability of guanacoite is mainly due to Mg-Cu substitution on the M1 and M2 sites.

Key Words: Guanacoite • copper magnesium arsenate hydrate • crystal structure • chemical analysis • Jahn-Teller distortion






JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Copyright © 2008 by Mineralogical Society of America