The minerals from Monte de São Bartolomeu, Portugal

Monte de São Bartolomeu (MSB), also known as Monte de São Brás, was classified national geological monument in 1979.

The outcrop of MSB (a volcanic chimney) is situated in a tiphonic valley at the east of the picturesque seaside village of Nazaré; beside there are other two smaller outcrops surrounded by sand dunes.

From its top, 156 meters above sea level, we can see a huge patch of pines and sand dunes; on the opposite side, the blue sea cut by the white houses of Nazaré. At the top there is a small chapel, a place of secular pilgrimage held annually in February, and an observation tower for forest fires.





































There are two ways to reach the top: at the east by a steep staircase following the contours of large rocks, or more hidden on the opposite side by a steep and uneven path under typical Mediterranean vegetation and other endemic species.

The geology of MSB is igneous. MSB resulted from an ascending magma, dating from the Upper Cretaceous, which resulted in a subophitic gabbro medium grain that formed at some depth and became outcropping due to the erosion of surrounding rocks. Like other igneous structures that occur in diapirically areas of the Lusitanian Basin, the eruption domes have established taking advantage of a fracture that extends through the Caldas da Rainha diapirically area from Leiria to Óbidos.



























According to prof. Cotelo Neiva (1948-49), the rock would be a subophitic gabbro, with veins of augite and microsienites. The subophitic gabbro texture of MSB is of medium grain, but in some points, literally becomes into a fine-grained facies. Overall this is a solid rock having augite and labradorite as primary minerals, as accessory minerals, biotite, sphen (titanite), zircon, rutile and apatite, and as accidental minerals occur pyrite and chalcopyrite; the secondary minerals of hydrothermal alteration, are green hornblende, hastingsite, biotite, chlorite, rutile, prehnite, quartz, calcite, kaolinite, magnetite, hematite and ilmenite.

The study of fine-grained facies showed a grayish dark green solid rock. Under the scope were observed an holocrystaline rock, a subophitic texture, fine grain, and as essential minerals, labradorite-andesite, augite, hornblende and biotite, as accidental mineral, apatite, rutile, zircon, magnetite, pyrite and sphen (titanite) and as secondary minerals, chlorite, quartz, calcite and prehnite.

At the southern base a microsyenite augite vein has been observed. It is a gray compact rock, very finely crystalline. In its gray background were noted small crystals of augite. The primary minerals are the predominant orthoclase, augite, magnetite and rutile, the accessory are albite-oligoclase, hornblende and hematite and the accidental are quartz and calcite. The secondary minerals are hornblende, chlorite and rutile.



I have been a frequent visitor of MSB over the last years and collected numerous small specimens with feldspar, hastingite, hematite, hornblende, magnetite, prehnite, quartz, titanite and others which require some analysis. Are somewhat common, and visible to the naked eye, nice twinned feldspar crystals, prehnite fans, hematite blades, hyaline quartz crystals up to 1 cm and beautiful octahedral crystals of magnetite from 1 to 3 mm.




During my first visits the above referred minerals were picked up right at the foot where the rocks were less altered and the specimens of better quality but, due to the improvements of the accessibilities, they are now underground and not easy to find anymore. At the top the minerals are greatly altered and rocks without cavities.
Today, there are still some places where, with some luck, you are able to find good specimens, although they were not easy to discover by who does not have a reasonable knowledge of the place.

Some information was obtained and freely translated from “Carta geológica de Portugal, notícia explicativa da folha 26-B Alcobaça, por J. Camarate França e G. Zbyszewski, geólogos dos serviços geológicos de Portugal, 1963”.

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