Although the Fogg doesn't date the work, the very unusual nature of the painting with the heavy halo effect may  put in line with some of the drawings he did for Alma Strettell's book Spanish and Italian Folk Songs

Bibliography

John Singer Sargent, His Own Work (New York, NY, 1980), in checklist, unpaginated

Alison Weber, "The Fortunes of Ecstasy: Teresa of çvila and the Discalced Carmelite Reform", Harvard Divinity Bulletin, vol. 28, no. 4, pp. 11-15, repr. on cover, repr. p. 11, repr. pp. 12, 15 (details)

Exhibitions

Sargent in the Studio: Drawings, Sketchbooks, and Oil Sketches, Harvard University Art Museums, Cambridge, 06/10/1999 - 09/05/1999