ENTROPY-DRIVEN DISORDER IN END-MEMBER AMPHIBOLES
FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2
Abstract
The synthesis and characterization of end-member monoclinic amphiboles are reviewed, with
a focus on chemical composition and order-disorder of cations over nonequivalent sites in the
amphibole structure. Synthetic end-member amphiboles that have been characterized by physical or
chemical methods in addition to optical examination and powder X-ray diffraction are tremolite, fluor-tremolite,
actinolite, pargasite, fluor-pargasite, M3+-substituted pargasite and fluor- pargasite (M3+ = Al, Cr3+, Ga, Sc),
ferro-pargasite, magnesio-hastingsite, hastingsite, richterite, potassium-richterite and ferro-richterite. In
none of these cases was an ordered end-member amphibole of ideal composition synthesized;
the amphiboles are either off-composition, or show unusual cation disorder, or both. Where
analogous natural compositions occur, they seem to show similar features. These unusual schemes of order
or unusual compositions are not present (or as prevalent) in synthetic and natural amphiboles of intermediate
composition. Such intermediate compositions are also characterized by Mg-Fe2+ and Al-Fe3+ order-disorder relations
that provide a significant contribution of configurational entropy to the energetics of
the structure, a contribution that is absent from an ordered end-member amphibole of ideal composition.
I propose that end-member or close-to-end-member amphiboles commonly show entropy-driven disorder that is
required for their occurrence as stable structures. This disorder may occur as unusual site-populations,
as small deviations from ideal end-member composition, or possibly as stacking disorder.
Keywords: amphibole, synthesis, order-disorder relation, entropy.