MINERALOGY AND PETROGRAPHY OF PHOSPHATE MINERAL ASSOCIATIONS FROM THE JOCÃO PEGMATITE, MINAS GERAIS, BRAZIL
Bibliographic record
Abstract
The Jocao pegmatite (or Cigana) is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. According to their macroscopic textures, three different kinds of phosphate mineral masses were collected from the dumps of the pegmatite: association I is composed of dendritic triphylite, forming intergrowths with silicate minerals (spessartine garnet or albite); association II forms blocky nodules of triphylite-ferrisicklerite-heterosite; and association III shows exsolution lamellae of ferrisicklerite-heterosite in massive beusite. The primary textures and the Fe/(Fe+Mn) ratios of primary triphylite make it possible to establish the crystallization sequence of the primary phases; the petrogenesis of primary intergrowths between garnet and triphylite of association I is also discussed. In the three associations, these primary minerals are hydrothermally altered and secondary species are produced. In association I, the first hydrothermal alteration event was a weakly oxidizing hydroxylation stage, during which triphylite was only replaced by hureaulite (rarely associated with barbosalite) along its cleavage planes. The final stage affecting association I corresponds to meteoric processes during which ludlamite and then vivianite progressively replaced triphylite. Association II evolved under more oxidizing conditions and the first alteration stage corresponds to the progressive oxidation of triphylite accompanied by Li-leaching, leading to ferrisicklerite and heterosite. The second hydrothermal stage corresponds to a hydroxylation event, and the secondary species depend on the phosphate mineral that they replace: triphylite is only altered to Fe 2+ -Mn 2+ -bearing hydrated species (colorless hureaulite); ferrisicklerite is altered to Fe 2+ -, Mn 2+ -, and Fe 3+ -bearing phosphate minerals such as jahnsite s.l. , frondelite s.l., and orange hureaulite; heterosite is replaced by Fe 3+ -bearing species such as ferristrunzite. Consequently, it appears that the secondary phosphate minerals, which crystallize during this second hydrothermal stage, strongly depend on the cations which are locally available in the sample zone. The final stage forms the meteoric species: leucophosphite and phosphosiderite directly replace heterosite, whereas vivianite, a ferrous meteoric species, only appears in the triphylite core. In association III, exsolution lamellae are formed at the expense of a high-temperature homogenous Ca-Li-bearing graftonite-beusite-like phase; when the temperature decreased, Li migrated into triphylite and Ca to the larger M1 site of beusite. During the high-temperature hydrothermal alteration processes, triphylite transforms into ferrisicklerite and heterosite, like in association II; however, beusite is not affected by any transformation process at that stage. During the low temperature hydroxylation stage, ferrisicklerite from the core remains almost unaltered, while beusite is replaced by an intimate mixture of pleochroic Ca-rich hureaulite and tavorite. After this hydroxylation stage, the meteoric stage is characterized by an increase in Ca 2+ and H 2 O activities, responsible for the replacement of hureaulite and tavorite by mitridatite-robertsite. At the nodule border, beusite and heterosite are completely replaced by an intimate mixture mainly composed of frondelite and robertsite-mitridatite. Finally, this study shows that the small phosphate nodules from Jocao are more affected by oxidation than large nodules, thus indicating that the diffusion kinetics of hydrothermal fluids is relatively low in these phosphate nodules. As a consequence, large phosphate nodules show a typical zoning with triphylite (core)–ferrisicklerite-heterosite (rim); this zoning, which preserves the crystal structure of the phosphate minerals, was achieved during the high temperature hydrothermal transformations. The phosphate nodules consequently appear as a relatively closed system compared to the silicate matrix.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 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) | 0.001 | 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 teacher head, 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".