Quartz after anhydrite

From Brazil

This is the 100^th post in The Geologic Column. I very much appreciate the interest from readers and subscribers. I continue to shoot for variety, with a focus on minerals, tectonics, and other geologic topics, and I’ll keep trying to hit the sweet spot where the posts may be interesting to lay people as well as professional geologists. Thanks!

This specimen may look like a bad job of throwing ragged knives into a bowl, but it’s actually a collection of former anhydrite crystals in a cavity in a basalt.

The rock is from Rio Grande do Sul, Brazil, where the Parana basalt famously is lined with amethyst crystals. Occasionally, the vugs (the cavities) in the basalt ended up with other minerals. Anhydrite is calcium sulfate, identical to gypsum except that gypsum has water in its crystal structure while anhydrite does not. Anhydrite’s name means “without water.”

It seems that in some places where the basalt was present layers of evaporites were also present. Evaporites including halite (common salt), gypsum, and anhydrite are deposited typically in restricted pools or shallows where the mineral salts first concentrate in the water and eventually precipitate out. These minerals are typically fairly soluble, so if the area became wet again after the minerals crystallized, the chemicals could move into the groundwater, from which they were occasionally redeposited in the open cavities in the basalt. An alternative origin for anhydrite in basalt is alteration of the basalt in contact with seawater.

These anhydrite crystals were quite large, up to almost 11 cm long with many of the flat blades around 6 or 7 cm. And they are no longer anhydrite. Sometime later in the story of this cavity, different mineralized waters came in, this time rich in silica. Quartz faithfully replaced the anhydrite blades, making this a pseudomorph (“false form”) of quartz after anhydrite. Sometimes quartz encrusts anhydrite, and the anhydrite then dissolves leaving a hollow mold of the anhydrite crystals, but in this case, it appears that the quartz actually replaced the anhydrite except for a couple small anhydrite crystals that are now gone but indicated by hollow molds within quartz.

The green color is the mineral celadonite, potassium iron-magnesium silicate. It’s a phyllosilicate (mica group, without the micaceous habit) that commonly alters from the iron and magnesium minerals in the basalt. It often deposits as a lining in the cavities in the basalt, and here it is also included in some of the quartz that also lines the cavity. Elsewhere in the specimen its color in the substrate is transmitted through the quartz crystals making them appear greenish, and in a few places, celadonite coats the quartz crystals on the cavity lining. “Celadon” is a pale green color named for a character Céladon who wore clothing of that color in a French story, Honoré D'Urfé's L'Astrée, a popular romance about fifth-century Gallic shepherds published in several instalments between 1607 and 1627. 

The alteration that produced the celadonite lining in the basalt vugs was caused by artesian water of the Guaraní and other aquifers percolating upward into the basalts (Marteani and others, 2010, The genesis of the amethyst geodes at Artigas, Uruguay, and the paleohydrology of the Guaraní aquifer: structural, geochemical, oxygen, carbon, strontium isotope and fluid inclusion study: Int. J. Earth Sci., Geol. Rundschau, 99:927).

Finally, there must have been some iron remaining in the waters (or in later waters) that percolated through the cavity. To the naked eye it looks like the quartz has dirt on it in places, but microscopically you see that those spots are actually tiny needles and clusters of elongate brown goethite crystals. Goethite is a common iron oxide-hydroxide named for the poet Goethe.

The crystals may look at first glance as if they have been glued into the cavity, but I’m confident that this specimen is entirely natural and has not been repaired. Acquired June 2021 at the Butte Mineral Show.

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Two great episodes of volcanism took place during the Cretaceous, one early in the period, and one at the end. Today’s topic is the earlier one.

In South America and adjacent parts of southern Africa (to which South America was connected) about 128 to 138 million years ago, thick basaltic flows poured into the Parana Basin. The Parana Basin was a persistent low-lying zone on the South American continent, located in southwestern Brazil and nearby areas, where the Parana River flows today. In some ways it was like the intracratonic basins of North America, such as the Williston, in that they were initiated by broad crustal warping early in the Paleozoic, probably during Ordovician time. But the Parana Basalts definitely make it a different beast from the sedimentary Williston Basin.

Present-day South Atlantic showing features related to Tristan Hotspot and South Atlantic Ocean opening.

The volcanism was most likely related to rifting that marked the onset of active seafloor spreading as the South Atlantic Ocean began to form. Rifts, faulting that breaks continents, often extends deep enough into the crust to allow deep-seated magma to reach the surface. One reason we are pretty much certain of the rift origin is that there is a conjugate pile of volcanic rocks on the African side of the same age as the Parana basalt, in Namibia and Angola, where they are called the Etendeka Volcanics. Furthermore, there are two lines of seamounts, underwater volcanoes, that extend from the coasts of both South America and Africa to the Mid-Atlantic Ridge. Those seamounts, called the Walvis Ridge on the African side of the Atlantic and the Rio Grande Rise on the South American side, extend essentially to the Mid-Atlantic Ridge at Tristan da Cunha, an active volcanic island whose magma source is probably the Tristan Hotspot.

Tristan da Cunha is pretty much just like Iceland, which also straddles both the Mid-Atlantic Ridge and a hotspot. One difference in addition to size (all hotspots are not created equal) between tiny Tristan da Cunha and massive Iceland is that the Tristan hotspot has been active sporadically since the Cretaceous whereas the Iceland one is quite recent, more voluminous, and has not yet split into two rows of seamounts. The two ridges that extend away from the Mid-Atlantic Ridge, and the volcanic piles on land in the Parana and Etendeka Provinces, reflect the episodic volcanism from the Tristan Hotspot as the Atlantic Ocean opened.

Some estimates put the Parana-Etendeka’s biggest eruptions, some of them ash eruptions rather than basalt flows, about 132 million years ago, as some of the largest volcanic events in earth history. Estimates of the volumes of at least five of these eruptions are two to four times the volume of the largest known Yellowstone supervolcano eruptions. The Parana-Etendeka basalts cover about 1.5 million square kilometers, about the same as the area of Montana, Wyoming, Utah, Idaho, Oregon, and Washington combined (a bit more than Australia’s Northern Territory plus Tasmania).

The Parana flood basalts in Rio Grande do Sul province, Brazil (and nearby Uruguay), are famous for their beautiful amethyst-lined tubes and geodes. Amethyst is purple quartz whose color is related to trace amounts of iron in the silica crystal framework, together with some degree of natural radiation and/or heat. The occurrences in the Parana are unusual in their intense color and uniformity, and for the huge size of some of the cavities. The geode formation was probably a two-stage process, with the openings forming initially in the original molten rock because of blobs of material (gas or liquid) that could not be incorporated into the basalt magma. Later, the cavities were the sites of deposition of quartz tainted by iron and other elements, including some rare-earth elements. The entire process of deposition of the amethyst probably took place over a time span on the order of 40 million years in the early to mid-Cretaceous.

Today, these amethysts make Brazil the leading producer of gem-quality amethyst in the world, and of course the huge amethyst-lined pipes are famous around the world as collectibles as well. For more technical information about the origin of these geodes, see for example Glig and others, 2003, Genesis of amethyst geodes in basaltic rocks of the Serra Geral Formation, Ametista do Sul, Rio Grande do Sul, Brazil: a fluid inclusion, REE, oxygen, carbon, and Sr isotope study on basalt, quartz, and calcite: Mineralium Deposita 38, 1009–1025.