Large-Scale Development of Supergene Anatase-Rich Soils on Carbonatite-Associated Pyroxenites in Brazil (Abstract)
Bibliographic record
Abstract
Abstract The climatic conditions in central and northern Brazil are conducive to the development of deep lateritic weathering in some carbonatite complexes, including the carbonatite rocks and some of their genetically and spatially related silicate rock units. Lateritic weathering of bebedourite-type pyroxenites genetically related to carbonatites in Brazil has produced a spectacular accumulation of high-grade anatase derived from the decalcification of perovskite. Carbonatite occurrences that contain vast quantities of supergene anatase-rich soils overlying weathered pyroxenites include Tapira, Serra Negra, Salitre I, and Salitre II in Minas Gerais; Cataläo I, in Goiás; and Maicuru and Maraconai in Pará. Soils overlying the weathered pyroxenites range from approximately 15 to 30 wt percent TiO2. At Serra Negra, an exploration drill hole gave an average of 20 wt percent TiO2 as anatase mineralization for a depth of 150 m. The average depth of weathering in weathered pyroxenite in the northwest quadrant of the Serra Negra circular structure is 120 m and the development of anatase is reported to be 40.5 million tons (Mt) of 28.52 wt percent TiO2. In the Bananeira area of Salitre I, the average depth of weathering is 80 to 100 m, with reported 60.34 Mt of 24.5 wt percent TiO2. In the Para carbonatite, the weathering depth may exceed 300 m. Within the weatheredpyroxenites, diopside, perovskite, apatite, and calcite undergo total chemical breakdown. Calcium and magnesium are efficiently removed from the complexes in the groundwater. Decalcification of the perovskite produces polycrystalline aggregates of micrometer-size anatase platelets. Liberated REE from perovskite, apatite, and calcite are largely immobile, forming supergene monazite, rhabdophane,cerianite, and crandellite-group minerals that occur as crystallites that are inextricably associated with anatase. Direct indication of the development of anatase from perovskite is clearly shown in three ways: (1) by texture with residual cores of perovskite surroundedby polycrystalline clusters of anatase, (2) by pseudomorphsof anatase after euhedral perovskite octahedral, and (3) because chondrite-normalized REE plots for anatase and perovskite show identical slopes that serve as a geochemical proof that anatase is derived from the perovskite. Bebedourite, first described by E. Tröger in 1928, is a pyroxenite containing major amounts of perovskite. The bebedouritesof Minas Gerais and Goiás are of Cretaceous age. Similar pyroxenites occur in other carbonatites, including the Precambrian Powderhorn of Colorado, North America, and in Kovdor of the Kola Peninsula, but in these occurrences, the absence of deep-penetrating lateritic weathering produces only a very thin rock-surface coating of anatase, for which the origin from the decalicification of perovskite is virtually identical to the anatase found in the Brazil carbonatites.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".