Red topaz from San Luis Potosi, Mexico, is well known among collectors. The red color is usually inferred to come from needle- and thread-like inclusions of rutile, titanium dioxide.
There seems to be some debate about the inclusions, which is surprising to me for such a famous example, but you’ll see descriptions of inclusions of rutile or hematite, probably rutile, rutile and hematite, and other equivocating phrases, and it’s also been suggested that the inclusions might be rings of goethite, iron oxide-hydroxide, together with open channels lined with some iron oxide (Gilg and Thomas, 2011, The inside story on topaz: in Topaz, Perfect cleavage, Clifford and others, eds., extra‐Lapis No. 14, 88‐93). It’s surprising that someone hasn’t done a truly definitive study to determine this, but if they have, I couldn’t find it.
Nonetheless, most descriptions say rutile.
The rhyolitic lava flows that contain the topaz erupted about 33 to 27 million years ago, part of a voluminous outpouring of lava in central Mexico that was probably related to extension of the crust rather than typical oceanic subduction and collision.
Topaz is Al2SiO4F2, simple aluminum silicate, but the F, fluorine, on the end of the formula is what makes it topaz. The fluorine also implies high temperatures, which tend to enrich fluorine in rocks at the expense of water (Burt and others, 1982, Topaz rhyolites; distribution, origin, and significance for exploration: Economic Geology 77:8 p. 1818-1836). The rhyolite as a whole is likely the result of partial melting of continental crust.
The topaz may have crystallized in cavities in the rhyolite directly from high-temperature vapor, rather than from liquid (Torres- Hernández and others, 2014, Geochronology and geochemical characteristics of a set of Tertiary rhyolitic domes in the San Luis Potosi Volcanic Field, Mexico: Boletín de la Sociedad Geológica Mexicana 66(1):183-197). That’s the likely origin of sharp garnets in similar rhyolites in Nevada and elsewhere.
It’s reasonable that the inclusions might have been thought to be hematite, which is rust-red when powdery or in thin strands, but rutile can certainly make red colors too – its name is from Latin rutilus, meaning "reddish." And rutile included in quartz is well known (rutilated quartz). Rutile is, I think, the most generally accepted identification for the needles in red topaz.
The name topaz has an ancient heritage, from Topasos Island in the Red Sea offshore Egypt, known today as Zabargad or St. John’s Island. But the original “topaz” of antiquity was probably applied to gemmy green olivine, peridot. Today’s mineral topaz received the name in 1737, applied to specimens from Germany.
interesting and informative! Thanks!
Richard, you've seen Frank Riehlicke's presentation on micromount zoom meeting I assume?? All the ring forming sulfides are Pb& Sb sulfs, except pyrite, (which wasn't the same kinda ring at all, I'd say). He said it was rutile at Potosi, you might ask him if any were tested for absence of Pb at least??
Could it be colored by one mineral while the rings are another? I bought one when I was into larger stuff. Got a scope, and much later realized I had a ring in my Potosi topaz. Just using a scope enlarges your collection.