A contribution to the study of the silicate karst in Sahara
Prilog proučavanju silikatnog krasa Sahare
Apstrakt
Silicate sandstones of Paleozoic age are very widespread in Sahara. In these were formed numerous caves and other relief forms, proper to the karst. In order to establish the genesis of these forms, a series of laboratory experiments have been made on the sandstone samples. The samples originate from the oasis Bardai (Tibesti, Chad), and also from a locality called Tamega, north of the border crossing In Guezzam (Algeria). By Bardai-sandstone has been analyzed the compact rock (test 1), the dust from decomposition, scraped oft from the rock (test 2) and the dust from the 'Quatre roches' foothill (test 3), and by Tamega-sandstone the compact rock (test 4) and dust from decomposition, scraped off from the rock (test 5). The rock samples were first reduced to dust, the size of its corns being under 0,5 mm. After that, from each sample were taken 5 g and put into a laboratory cup of quartz glass in which there already were 2 l distilled water. At the temperature of 21-22.5 the pump blew co...ntinually for 15 days air into water with a view to forming H2C03 more rapidly and possibly to dissolving mineral matter. In parallel with this proceeding, there was established the quantity and chemical structure of cement in all the samples (Tab. 2). This was done by dissolving the pulverized sample in boiling distilled water, which was chemically analyzed after that. In the first experiment, concentrations of individual elements after 24 hours of dissolving reached already 50-70% of values obtained after 15 days of dissolving (Tab. 1). This points out that the process of dissolving and of chemical modification of mineral substance is comparatively rapid and that it depends more on the quantity of fresh water and on the temperature, than on the length of contact between water and rock. It was established further, that (lie disintegration of analyzed sandstones occurs chiefly on account of the transition of calcium, sodium and potassium from cement into the water solution. Although the grains of quartz in sandstones constituted over 95% of the rock mass, in water were found very small quantities of dissolved silicates - negligible in comparison with other chemical components. Microscopic analysis of the Tamega-sandstones, which are of Ordovician age, it turned out that the quartz grains in them were exposed to the processed of metamorphism and even before their sedimentation, owing to transport, partly processed. Consequently, there are no proofs that the 'corrosive' changes on the quartz grains, which were observed by some investigators, are in relation with geomorphologic processes which have led to the creation of forms of silicate karst. On the basis of the executed laboratory experiments, one came to the conclusion that the forms of silicate karst are polygenetic and only morphologically similar to the forms of relief which are formed in limestone, dolostone and gypsum.
Izvor:
Glasnik Srpskog geografskog društva, 1991, 71, 1, 21-28Izdavač:
- Beograd : Srpsko geografsko društvo
Kolekcije
Institucija/grupa
Geografski fakultetTY - JOUR AU - Gavrilović, Dušan M. AU - Manojlović, Predrag PY - 1991 UR - https://gery.gef.bg.ac.rs/handle/123456789/9 AB - Silicate sandstones of Paleozoic age are very widespread in Sahara. In these were formed numerous caves and other relief forms, proper to the karst. In order to establish the genesis of these forms, a series of laboratory experiments have been made on the sandstone samples. The samples originate from the oasis Bardai (Tibesti, Chad), and also from a locality called Tamega, north of the border crossing In Guezzam (Algeria). By Bardai-sandstone has been analyzed the compact rock (test 1), the dust from decomposition, scraped oft from the rock (test 2) and the dust from the 'Quatre roches' foothill (test 3), and by Tamega-sandstone the compact rock (test 4) and dust from decomposition, scraped off from the rock (test 5). The rock samples were first reduced to dust, the size of its corns being under 0,5 mm. After that, from each sample were taken 5 g and put into a laboratory cup of quartz glass in which there already were 2 l distilled water. At the temperature of 21-22.5 the pump blew continually for 15 days air into water with a view to forming H2C03 more rapidly and possibly to dissolving mineral matter. In parallel with this proceeding, there was established the quantity and chemical structure of cement in all the samples (Tab. 2). This was done by dissolving the pulverized sample in boiling distilled water, which was chemically analyzed after that. In the first experiment, concentrations of individual elements after 24 hours of dissolving reached already 50-70% of values obtained after 15 days of dissolving (Tab. 1). This points out that the process of dissolving and of chemical modification of mineral substance is comparatively rapid and that it depends more on the quantity of fresh water and on the temperature, than on the length of contact between water and rock. It was established further, that (lie disintegration of analyzed sandstones occurs chiefly on account of the transition of calcium, sodium and potassium from cement into the water solution. Although the grains of quartz in sandstones constituted over 95% of the rock mass, in water were found very small quantities of dissolved silicates - negligible in comparison with other chemical components. Microscopic analysis of the Tamega-sandstones, which are of Ordovician age, it turned out that the quartz grains in them were exposed to the processed of metamorphism and even before their sedimentation, owing to transport, partly processed. Consequently, there are no proofs that the 'corrosive' changes on the quartz grains, which were observed by some investigators, are in relation with geomorphologic processes which have led to the creation of forms of silicate karst. On the basis of the executed laboratory experiments, one came to the conclusion that the forms of silicate karst are polygenetic and only morphologically similar to the forms of relief which are formed in limestone, dolostone and gypsum. PB - Beograd : Srpsko geografsko društvo T2 - Glasnik Srpskog geografskog društva T1 - A contribution to the study of the silicate karst in Sahara T1 - Prilog proučavanju silikatnog krasa Sahare VL - 71 IS - 1 SP - 21 EP - 28 UR - https://hdl.handle.net/21.15107/rcub_gery_9 ER -
@article{ author = "Gavrilović, Dušan M. and Manojlović, Predrag", year = "1991", abstract = "Silicate sandstones of Paleozoic age are very widespread in Sahara. In these were formed numerous caves and other relief forms, proper to the karst. In order to establish the genesis of these forms, a series of laboratory experiments have been made on the sandstone samples. The samples originate from the oasis Bardai (Tibesti, Chad), and also from a locality called Tamega, north of the border crossing In Guezzam (Algeria). By Bardai-sandstone has been analyzed the compact rock (test 1), the dust from decomposition, scraped oft from the rock (test 2) and the dust from the 'Quatre roches' foothill (test 3), and by Tamega-sandstone the compact rock (test 4) and dust from decomposition, scraped off from the rock (test 5). The rock samples were first reduced to dust, the size of its corns being under 0,5 mm. After that, from each sample were taken 5 g and put into a laboratory cup of quartz glass in which there already were 2 l distilled water. At the temperature of 21-22.5 the pump blew continually for 15 days air into water with a view to forming H2C03 more rapidly and possibly to dissolving mineral matter. In parallel with this proceeding, there was established the quantity and chemical structure of cement in all the samples (Tab. 2). This was done by dissolving the pulverized sample in boiling distilled water, which was chemically analyzed after that. In the first experiment, concentrations of individual elements after 24 hours of dissolving reached already 50-70% of values obtained after 15 days of dissolving (Tab. 1). This points out that the process of dissolving and of chemical modification of mineral substance is comparatively rapid and that it depends more on the quantity of fresh water and on the temperature, than on the length of contact between water and rock. It was established further, that (lie disintegration of analyzed sandstones occurs chiefly on account of the transition of calcium, sodium and potassium from cement into the water solution. Although the grains of quartz in sandstones constituted over 95% of the rock mass, in water were found very small quantities of dissolved silicates - negligible in comparison with other chemical components. Microscopic analysis of the Tamega-sandstones, which are of Ordovician age, it turned out that the quartz grains in them were exposed to the processed of metamorphism and even before their sedimentation, owing to transport, partly processed. Consequently, there are no proofs that the 'corrosive' changes on the quartz grains, which were observed by some investigators, are in relation with geomorphologic processes which have led to the creation of forms of silicate karst. On the basis of the executed laboratory experiments, one came to the conclusion that the forms of silicate karst are polygenetic and only morphologically similar to the forms of relief which are formed in limestone, dolostone and gypsum.", publisher = "Beograd : Srpsko geografsko društvo", journal = "Glasnik Srpskog geografskog društva", title = "A contribution to the study of the silicate karst in Sahara, Prilog proučavanju silikatnog krasa Sahare", volume = "71", number = "1", pages = "21-28", url = "https://hdl.handle.net/21.15107/rcub_gery_9" }
Gavrilović, D. M.,& Manojlović, P.. (1991). A contribution to the study of the silicate karst in Sahara. in Glasnik Srpskog geografskog društva Beograd : Srpsko geografsko društvo., 71(1), 21-28. https://hdl.handle.net/21.15107/rcub_gery_9
Gavrilović DM, Manojlović P. A contribution to the study of the silicate karst in Sahara. in Glasnik Srpskog geografskog društva. 1991;71(1):21-28. https://hdl.handle.net/21.15107/rcub_gery_9 .
Gavrilović, Dušan M., Manojlović, Predrag, "A contribution to the study of the silicate karst in Sahara" in Glasnik Srpskog geografskog društva, 71, no. 1 (1991):21-28, https://hdl.handle.net/21.15107/rcub_gery_9 .