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American Mineralogist; February-March; v. 94; no. 2-3; p. 210-221; DOI: 10.2138/am.2009.3008
© 2009 Mineralogical Society of America
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Suhailite, a new ammonium trioctahedral mica

María Dolores Ruiz Cruz1,* and Carlos Sanz de Galdeano2

1 Departamento de Química Inorgánica, Cristalografía y Mineralogía, Universidad de Málaga, 29071 Málaga, Spain
2 Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, 18071 Granada, Spain

Correspondence: * E-mail: mdruiz{at}uma.es

A new ammonium-bearing trioctahedral mica (suhailite) has been found in gneisses from the Betic Cordillera (Spain). Suhailite appears as aggregates of golden grains unoriented with respect to the main foliation. It shows weak pleochroism from light to darker yellow and low birefringence (0.028). X-ray data indicate the presence of two compositional variations: a NH4-rich phase (with a basal spacing of 10.40–10.44 Å) and a NH4-K intermediate phase (with basal spacing of 10.20–10.26 Å). Average composition, as deduced from analyses obtained at the scale of the scanning electron microscope is [Ca0.04Na0.07K0.35(NH4)0.55](Al0.42Ti0.22Fe1.33Mn0.01Mg0.71){sum}=2.70(Si2.67Al1.33)O10(OH)2. Thermal data indicate that maximum NH4 detachment occurs at 502 °C, suggesting a thermal stability similar to tobelite. Textural data indicate that unoriented golden grains consist of fine intergrowths of annite and suhailite and suggest that suhailite formed from primary red annite during the annite to fibrolite transformation.

Key Words: Annite • Betic Cordillera • suhailite • SEM • TEM/AEM • XRD






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