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Tourmaline Classification Scheme
(as proposed by Hawthorne and Henry (1999))

Over the last several years it has become apparent that there is considerable confusion in the assignment of names to specific tourmaline compositions. There are several reasons for this confusion:

• Some of the formal descriptions of tourmaline minerals specifiy the ideal end-member compositions but do not specify the limits of the use of the name;

• Some of the formal descriptions of tourmaline minerals specify the general composition but do not specify the end-member composition;

• There has been a promulgation of the idea of the "50% rule" whereby a specific chemical component in a mineral can only give rise to a new species if it exceeds 50% occupancy of the site (or group of sites) at which it occurs;

• There has been a use of  a less-than-optimal graphical representations to show compositional variations in tourmaline.

These reasons prompted a re-examination of current end-member species and potential new end-members, and  lead to the development of several useful compositional diagrams.

Tourmaline can be separated into principal groups based on the dominant occupancy of the X site, the alkali-, calcic- and vacant-tourmaline groups. This general grouping makes petrologic sense in that X site occupancy are likely to reflect the paragenesis analogous to similar general groupings in the amphibole and pyroxene systems. Another important factor is that F partitions exclusively into the W site and O^2- also tends to partition to the W site. Thus, the dominant anion in the W site becomes the basis for a secondary series of possible hydroxy-, fluor- and oxy-species. In turn, the presence of dominant O^2- at the W site mandates that local cation ordering take place among the cations at the Y and Z sites

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There are currently 14 tourmaline end members that have been accepted by the International Mineralogical Association (IMA). In the Hawthorne and Henry (1999) paper, we re-examined the compositions of the holotype material and, in some cases, redefined the end member compositions (Table 1).
 

Table 1. Current tourmaline end member species accepted by IMA

Alkali tourmaline

Calcic tourmaline

X-site vacant tourmaline

* These end-members are modified from the original suggested formulae to produce proper end-members (Hawthorne and Henry, 1999).

Hawthorne and Henry (1999) point out that due to the chemical diversity and structural requirements of tourmaline there are a large number of additional hypothetical end member species that may exist, but need to be verified (Table 2).

Table 2. Additional hypothetical tourmaline end member species inferred from probable site occupancies (not currently recognized by IMA)

Alkali tourmaline

Calcic tourmaline

X-site vacant tourmaline

**These endmembers are theoretically possible, but not likely due to X-site vacancy - F avoidance that appears to take place in tourmaline.
 

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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