The yellow iron minerals of Rio Sucio in Costa Rica

The merging of the Sucio and Hondura Rivers near the bridge of Route 32 is a must-see highlight on the scenic road through Braulio Carrillo National Park. The crystalline waters of the Hondura River meet the ocher-yellow waters of the Sucio River (meaning muddy or dirty) less than 100 m upriver from the bridge. The powerful currents create swirls and eddies that extend for dozens of meters, clearly delineated by the colored waters, before going under the bridge. On the background the two river canyons disappear into the virgin rainforest.

The Sucio River on the sampling day in 2014 . Incredibly, no one had explored the river canyon by then.

The sediment load of the Sucio River is constant throughout the year, becoming less noticeable when the heavy rains come. We know that its sources are on the northern flank of Irazu volcano at an altitude of about 3000 m. Ever since I saw it for the first time in the early 90’s I wondered about the yellow material’s chemical nature and its relationship to the volcano but I knew it would require more than simple inorganic chemistry to find out.

The clear waters of Hondura River mix with Sucio River as seen from the bridge on Route 32.

A few years ago I met bio technologist and molecular biologist Max Chavarría who had recently joined our faculty. Max wanted to know if I could suggest places where extremophiles (microbes that live in extreme environments) could be found for his research… and of course Río Sucio came up first in our conversation.

A few weeks later we were parked by the Sucio River bridge, equipped with sterile containers for collecting water and sediment samples and we took a short trail to the water’s edge. The rocks were covered by layers of yellow, orange and red iron minerals and the yellow waters rushed by on their way to meet the Hondura River.

Upon exposure to the tropical sun the suspended minerals dehydrate and form a hard coating on the boulders.

Just from the observation of the mineral deposits going from soft yellow and orange to hard red mineral layers away from the current it was evident that a dehydration process was taking place that probably ended in hematite formation… but what was the suspended yellowish material?

Filtering or letting settle the water samples readily afforded the solid which was not a simple ferric hydrous oxide as the IR spectrum clearly showed sulfate bands. Initially I didn’t expect to see any Powder X-Ray diffraction pattern as the substance was clearly amorphous so I thought it would be a dead end.

After quite a bit of searching and reading (I am not a mineralogist after all) it seemed that the solid could belong to a group of yellow iron oxides that incorporate other anions in their structure. I first found about Akaganeite which contains chloride anions that can be replaced by others such as sulfate. This would make sense as sulfuric acid is present in volcanic acidic water sources. The paper also described that even though these solids are amorphous they can show weak and broad diffraction patterns.

Where the hardened layer formed on top of sand, water eventually carved underneath.

I then took the samples to Leonardo Rojas whom at the time was in charge of the Powder X-Ray Diffractometer. Now he has joined the Inorganic Chemistry Group after getting his doctorate in Germany. Leonardo then set up a long data collection routine that gave us the first diffraction pattern of the Río Sucio mineral: It resembled sulfate-substituted Akaganeite.

It used to be that only minerals that were crystalline were accepted as such in the mineral databases. However, with the advent of new techniques and computational methods that can get structural information from powder diffraction patterns, amorphous substances such as these iron oxides are now recognized as minerals in their own right. Some further search into the literature finally revealed that the yellow sulfate containing iron oxide from the Sucio River is the mineral Schwertmannite, named after iron oxide expert U. Schwertmann and only included in the mineral databases a few years ago.

The Sucio and Hondura Rivers on a sunny day. A similar viewpoint was used on some takes of the 1995 Paramount’s movie Congo.

Now the mystery of the Sucio River was finally solved… or was it?

It turns out that Schwertmannite forms in Acid Mine Drainage waters where bacteria take advantage of the oxidation of pyrite to gather metabolic energy, and produce sulfuric acid and ferric iron in the process. If the acidity and sulfate concentrations are just right then this mineral is produced. If the acidity and sulfate levels are lower other iron oxides form. These pyrite-oxidizing bacteria thrive in very acidic waters and are thus termed acidophiles.

So, here is where Max had to provide some answers: were there extreme acidophilic bacteria in the water? If there were, then we would have a natural (and huge!!) analogue of an acid mine drainage site. It turned out there are extremophiles …but this is a subject for another post!

Post by Eduardo Libby

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