G is for Glaucophane

Mine are darker than “sky-blue”

Glaucophane and garnets, Laytonville Quarry, California USA. In all the photos, the garnets are about 0.1 to 0.3 mm in diameter.

Glaucophane is a blue amphibole, sodium-magnesium-aluminum silicate, [Na₂][Mg₃Al₂]Si₈O₂₂(OH)₂.  

It’s relatively unusual because it crystallizes under low-temperature (200–500 °C or 392–932 °F) but high-pressure conditions (depths corresponding to 15–30 km or 9.3–18.6 mi), usually in metamorphic rocks in subduction zones that contain considerable water. Rocks containing glaucophane are called blueschists for the color the mineral gives them.

Glaucophane and blueschists usually form by metamorphism of basaltic rocks, such as those that make up oceanic crust, so it’s no surprise that they form in subduction zones. It’s a little more surprising that they reach the surface, but in complex tectonic zones like the Coast Ranges of California USA, a lot can happen, and such rocks have reached the surface there in a diverse package of volcanic rocks, sediments, and crustal material called the Franciscan Assemblage.

“Rivers” of garnets in fine-grained glaucophane blueschist.

At the Laytonville Quarry in Mendocino County, California, part of the Franciscan package is a small body, originally only about 40 meters long, that’s “exotic” even for the Franciscan. I think the standard interpretation of this exotic block is that it was a localized body of iron-rich sediment (even ironstone) on the deep sea floor, associated with more standard chert and carbonate sediments.

When the block was metamorphosed along with the rest of the package, abundant glaucophane and garnets formed. The garnets are mostly in the almandine-spessartine range, iron-aluminum to manganese-aluminum. You can’t go absolutely by color for garnet species, but based on similarities with analyzed specimens, the deep red to red-orange garnets in my photos here are probably toward almandine and the lighter, more yellow-orange crystals are toward spessartine.

Glaucophane in my specimens here ranges from dark blue, almost black, crystals to lighter blue to fine meshworks with a silvery sheen. There’s a chance that some of these crystals might be riebeckite (with iron rather than glaucophane’s magnesium-aluminum) but I don’t think so.

The little block at Laytonville is also the type locality for three minerals, deerite, howieite, and zussmanite, three unusual iron-manganese silicates (relatively depleted in aluminum) described in 1965 and named for the three co-authors of a popular mineralogy textbook, An Introduction to the Rock-Forming Minerals (Muir Wood, 1982, The Laytonville Quarry (Mendocino County California) exotic block: iron-rich blueschist-facies subduction-zone metamorphism: Mineralogical Magazine, v. 45, p. 87-99).

Franciscan Complex drawing modified slightly from National Park service (public domain), original graphic by Trista Thornberry Ehrlich.

The Franciscan rocks are mélanges (from French for mixture) of rocks that were disrupted both during sedimentation (as huge submarine slumps and slides) and by later tectonic activity during subduction and even later upheaval (Raymond, 2019, Origin of mélanges of the Franciscan Complex, Diablo Range and Northern California: An Analysis and Review: Geosciences 9, no. 8: 338).

Specimens all collected in 1972 on a geology field trip from the University of California at Davis. Glaucophane is from Greek γλαυκός glaukos for "sky-blue" and φαίνεσθαι phainesthai "to appear."