Giant Calcite in Portugal
An article by Alain Martaud, Chambéry, France
In 1986, within the framework of my geological studies at IGAL in Paris, France* my professors offered for me to work on an area of central Portugal. The intention was to make the geological map by myself and all the necessary studies to fill two reports: one at the end of the third year to validate the first step, one at the end of the fifth year to validate all my courses. The area is rather big: the Serras (ranges) dos Candeeiros and de Aire, the Planalto de Santo Antono and Sao Mamede, and all the adjacent basins. The landscape is made of limestone plateaus with karstic features and Mediterranean vegetation; the rocks go from triasic salt to quaternary continental sands; the main part of the ranges are made of Jurassic pure limestone and marls. So the subject seems to be very serious for somebody who went to geology as a mineral collector! Fortunately the cave potential is great and made me happy thinking about my second hobby: caving…
* Institut Géologique Albert de Lapparent, now Institut Polytechnique LaSalle Beauvais, engineer and technician school of agronomy, geology and agro-food.
A colleague of mine who had work nearby told me, "Go to the western foot of Serra Dos Candeeiros at the village of Moita Do Poco. You'll see an old quarry with big calcite crystals. The geologic context seems to be strange but you'll like it." Believe me, I ran! At the end of a trail there was a small abandoned quarry. The place looked like a "ball" measuring 120 x 80 x 50 meters high. Strata were visible but not related with the bedrock of Bathonian reef limestone. Going further I understood that those strata were made of metrics roots of calcite crystals making palisades! Just unbelievable to see such a thing for a geologist who had never heard or read anything about it. Geodes were seen from place to place between the "calcite strata" showing parallel growth of scalenohedrons and clay. I finally learned that the quarry had been artisanally worked from time to time since the mid 1960s by somebody who was carving the cleavages' rhombohedrons coming from the heart of the big crystals. Unfortunately I still have not seen any of these pieces of art. The way of processing the quarry created a dangerous overhang roof looking like the entrance of a giant cave of China. I only had hand tools and little time to collect. The purpose of the summers spent in Portugal was a geological map, fault and dipping of stratas measures, rock sample collecting for thinblade studies and more! I spent also much time caving. A lot of caves and potholes there were still "virgin" at that time.
I made a visit to the same quarry to show that unusual place to my future wife Caroline. A collapse of the roof showed two dozen of scalenohedrons (see photos below) from 30 to 80 cm long! Gosh, I thought, how to save something? We had no time, no tools, no money to spend and no room in the car; at that time it was still a two-and-a-half-day drive from Paris. So we took a few pictures and chose one undamaged crystal that we were able to carry. We also took a few bicolor rhombohedron cleavages (champagne and honey) perfectly gemmy, certainly the target of the former quarrymen.
Until 1992, we spent many months in Portugal, but I concentrated on looking for pegmatites in the north of the country when I was not working. I gave the address of the quarry to two people who gave it to a few other French prospectors but in the end not that much was found: a few good crystals, but nothing in an outstanding size range. In 1991, I wrote a paper for the RAST (Rassemblement des sciences de la terre/ French annual meeting of geosciences) about that locality. The few results of our work showed that those calcites filled a karstic system in the phreatic zone during the late Cretaceous or early Cenozoic era, using an early karstic system dating from the middle Oxfordian, system visible everywhere around (to make it simple...).
I'm finally back for a few days in the area, following the advice of my friend, the French prospector Michel Ambroise. Almost nothing had changed in the quarry, but near there are two new working quarries. One is of no interest but the other one on the opposite side of the little valley, about 400 m from there, shows some interconnected open pockets of several meters long. The average size is more usual—5 to 10 cm—but the quality is really better, more "fresh." Twins are visible from time to time and it was possible to unearth a few good pieces. In the upper part of the second quarry, the wall of ornamental limestone has been newly cut by the chainsaw. It was possible to see the rest of a recently "eaten" vug by the quarry activities (see photos below). As it was impossible to retrieve it in its entire size (I asked the help of the quarrymen, mentioning money, but they didn't seem to care) I decided to take a chance on the sole undamaged scalenohedron with only a two-kilo hammer and chisels. The second day, with patience and mainly by chance I was able to save a 25 cm crystal on matrix. Unfortunately the cleaning showed that the specimen was of poor quality, already weatherized: too bad. Looking around I'd seen in the dumps big broken crystals (40 to 90 cm) but of very good quality, having been protected by thick red clay: really too bad. Michel and his wife Arista were more lucky and found some regular-size but superb specimens, bright, gemmy, sometimes twinned.
During holidays with my family, Caroline and I made a "check" visit in the old quarry and the new one. Nothing had really changed and the quarry works were on another side. We met an old charming gentlemen and he was soon having a look at the old quarry. This man was the famous Portuguese geologist Galopim de Carvalho who later created the Museu do Quartzo (Quartz Museum) in Viseu. One week later I decided to drive to see Professor Fernando Barriga, curator of the Geological Museum of Lisbon. As it is close to the regional natural park of Serras de Aire e Candeeiros we could hope something may happen in the future. Michel Ambroise used to check from time to time the working quarry but no good geodes had been found during those last years. Another working quarry not far away in Alto Da Serra produced some good calcite clusters to local collectors and also to French collector Sebastien Glineur.
This area is a nice target for holidays, not far away from the beach of Nazare, the monasteries of Alcobaça and Bathalha, the castle city of Obidos and many more. The food is fine and the people are unassuming and friendly!