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Tourmaline Crystal Structure

Tourmaline is an acentric rhombohedral borosilicate with a general chemical formula of XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W.

Structural components in tourmaline

• The structure of tourmaline is characterized by six-membered tetrahedral rings (T sites) whose apical oxygens point toward the (-) c-pole, producing the acentric nature of the structure.
  
• The tetrahedral T sites are primarily occupied by Si, locally with lesser amounts of Al and, possibly, B (Hawthorne et al., 1993; Tagg et al., 1999).
  
• Alternating over the tetrahedra are triangular BO₃ groups that lie parallel to the (0001) plane. Most of the crystallographic studies indicate that there are stoichiometric amounts of B in this site (e.g. Hawthorne, 1996; Bloodaxe et al., 1999).
  
• Planar rings of tetrahedra are linked by two types of octahedra, Z and Y, which share edges to form brucite-like fragments.
  
• The Z octahedron is relatively small, somewhat distorted, and occupied predominantly by trivalent cations such as Al³⁺, Fe³⁺, Cr³⁺ and V³⁺, but can contain significant amounts of the divalent cations Mg²⁺ and, possibly, Fe²⁺. Z site cations serve as linkages among structural fragments along a three-fold screw axis.
  
• The Y site is a relatively regular octahedral polyhedron occupied by a wide array of multivalent cations; most commonly Li¹⁺, Mg²⁺, Fe²⁺, Mn²⁺, Al³⁺, Cr³⁺, V³⁺, Fe³⁺ and Ti⁴⁺. Most structural refinements indicate that there is little or no Y site vacancy (e.g. Hawthorne et al., 1993; MacDonald et al., 1993; Taylor et al., 1995).
  
• The X site is a nine-coordinated trigonal antiprism, located along the three-fold axis of symmetry. It is most commonly occupied by Na and Ca (with minor K), or can be largely vacant.
  
• There are 31 anions in the structural formula that are located in 8 distinct sites, O(1) through O(8) (Donnay and Buerger, 1950; Donnay and Barton, 1972). The O(2) and O(4)-O(8) are occupied exclusively by O^2-.
  
• The single O(1) site, termed the W site in the structural formula, is located along the three-fold axis central to the pseudo-hexagonal ring of tetrahedra and can contain OH^1-, O^2- or F^1-. Significantly, where O^2- is dominant at the W site, short range ordering is commonly required at the Y and Z sites such that Mg commonly orders into the Z site and Al into the Y site (Hawthorne and Henry, 1999).
  
• The three O(3) sites, termed the V site, typically contain OH^1-, but can also contain significant amounts of O^2-. The W site is unique because F^1- partitions completely into this site, and O^2- also tend to partition into this site relative to OH^1- (Grice and Ercit, 1993; MacDonald and Hawthorne, 1995).

Darrell Henry is the Campanile Charities Professor of Geology and Geophysics at Louisiana State University whose research specialty is metamorphic petrology. Further details of his professional background are included in an accompanying vita or faculty profile.

To contact Darrell Henry call (225)-578-2693, fax (225)-578-2302 or e-mail dhenry@geol.lsu.edu . Address: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803.

This page was last updated on 04/17/06.

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