Petrographic Study of Some Indian Phosphorites

This paper incorporates results of textural and mineratogical study of phosphorites from Mussourie and Maldeota in Uttar Pradesh, and Kanpur, Maton and Birmania in Rajasthan. In Uttar Pradesh phosphorites, the phosphatic mineral collophane occurs in pelletal, interstitial and massive form. Different types of pellets - ovulitic, oolitic, encased, nucleated and polynucleated, are encountered. Most of the collophane is isotropic, rarely cryptocrystalline or microcryptocrystalline. Calcite, muscovite-sericite, clay, quartz and chert are the principal gangue associates. In phosphorite from Kanpur collophane lacks pelletal arrangement and occurs in the form of irregular bands in limestone. Maton phosphorites consist of nodules and large pellets of micro- and cryptocrystalline collophane enclosed in a matrix of chert and calcite, while in Birmania collophane occurs both in the form of dispersed pellets and bands in limestone.

Infra-red spectra of pure collophane samples from each of the localities show the presence of structural (OH) and (CO₃) groups. This coupled with the chemical analysis shows that the samples from Maldeota and Birmania can be called carbonate-hydroxyl-fluorapatite, while those from Kanpur and Maton fall in the range of fluoiapatite. Calculation of ionic composition of the collophanes shows that C replaces P, further the direct relationship observed between H₂O⁺ and Ca/P+C ratio indicates the possibility of PO«^*(OH)₄ substitution. The unit cell dimensions fall in between those of fluorapatite, hydroxylapatite and francolite. Substitution of C for P reduces slightly the average refractive indices.

It is interesting to note that collophane from Maton and Kanpur areas which occur in precambrian formations approach fluorapatite in composition while those from later horizons (Birmania - late Palaeozoic, Mussourie and Maldeota - Mezozoic) are carbonate-hydroxyl-fluorapatites.