A first attempt to use correlations in tourmaline as a thermometer

Author(s)

Andreas Ertl

Abstract

A new geothermometer is proposed based on the chemical composition of minerals from

the tourmaline supergroup, which have the formula XY₃Z₃(BO₃)₃[T₆O₁₈]V₃W. An excellent

correlation between the total charge of the X-site cations and the fluorine content in pegmatitic

tourmalines was first reported by [1]. The W site is bound to three cations of the Y site. Where

OH occupies the W site, the H points to the X site. The presence of F at the W site immediately

adjacent to the XO₉ polyhedron increases the bond length distortion of this polyhedron [2]. A

dependence of the X-site occupancy to the formation conditions during tourmaline

crystallization was already suggested by [3]. Over the last 15 years, several excellent

correlations between X-site charge and F content have been observed in tourmalines from

various pegmatites, such as that reported by [4]. On that correlation, all zones of a tourmaline

crystal from a miarolitic pocket that vary significantly in their chemical composition lie

perfectly on the straight line of a linear function. Therefore, such correlations seem to be

independent of the chemical composition but dependent on the formation conditions. A high

positive correlation seems to require equal formation conditions and an essentially closed

system (see also [4]). The linear function of tourmaline from different pegmatites usually has

different slopes. To develop a thermometer, tourmalines synthesized at 650 °C / 150 MPa [5]

and at 750 °C / 200 MPa [6] were used for calibration. The resulting formula for the tourmaline

thermometer is: (1.249 – k) ± 120 + 650 = T (°C), where k is the slope of the X-site charge to

F function. Initial results deliver consistent results: 592 ± 50 °C for a tourmaline from a

metapelite exposed to 630 ± 50 °C. The temperature of a tourmaline (schorl – fluor-elbaite

series) from a miarolitic cavity [4] was calculated as 706 ± 50 °C. Although this value agrees

with literature data (cited in [4]), it is not without controversy. The accuracy of this proposed

tourmaline thermometer is currently limited due to calibration points at only two different

temperatures. 

References:
[1] Ertl, A., Kolitsch, U., Meyer, H.-P., Ludwig, T., Lengauer, C.L., Nasdala, L. & Tillmanns,

E. (2009). N. Jb. Miner. Abh. 186, 51–61. 
[2] Ertl, A., Hughes, J.M., Pertlik, F., Foit, F.F.Jr., Wright, S.E., Brandstätter, F. & Marler, B.

(2002). Can. Mineral. 40, 153–162. 
[3] Henry, D.J. & Dutrow, B.L. (1996). Rev. Mineral. Geochem. 33, 503–557. 
[4] Ertl, A., Rossman, G.R., Hughes, J.M., London, D., Wang, Y., O’Leary, J.A., Dyar, M.D.,

Prowatke, S., Ludwig, T. & Tillmanns, E. (2010). Am. Mineral. 95, 24–40. 
[5] Ertl, A., Vereshchagin, O.S., Giester, G., Tillmanns, E., Meyer, H.-P., Ludwig, T.,

Rozhdestvenskaya, I.V. & Frank-Kamenetskaya O.V. (2015). Can. Mineral. 53, 209–220. 
[6] London, D., Ertl, A., Hughes, J.M., Morgan VI, G.B., Fritz, E.A. & Harms, B.S. (2006).

Am. Mineral. 91, 680–684.

Organisation(s)

Department of Mineralogy and Crystallography

Pages

38

Publication date

2024

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy, 105113 Crystallography

Portal url

https://ucrisportal.univie.ac.at/en/publications/65d40e3c-4fd8-48be-96dd-3f69d6a1f925