Compositional variation of piemontites from different Mn-rich rock-types of the Iberian Massif (SW Spain)
Jiménez-Millán, Juan; Velilla, Nicolas
European Journal of Mineralogy Volume 5 Number 5 (1993), p. 961 - 970
published: Jan 1, 1993
manuscript accepted: May 6, 1993
manuscript received: Oct 12, 1992
ArtNo. ESP147050505019, Price: 29.00 €
Abstract Four types of piemontite-bearing Mn-rich assemblage are studied from the Ossa-Morena Zone (Iberian Massif, SW Spain): silicate rocks containing braunite (SRB), silicate rocks containing hematite (SRH), carbonate rocks containing braunite nodules (CRB) and carbonate rocks containing thin hematite beds (CRH). These rocktypes are found in greenschist facies volcano-sedimentary units. High oxygen fugacity is crucial in the stability of piemontite in the studied assemblages. Variable piemontite end-member contents in the CRB and CRH rocks result from differences in the fO2 ; the molar proportion of piemontite in the CRB piemontites (ca. Pmπ) indicates higher JO2 conditions than those of CRH manganian epidotes (Pm6Ps24). The Fe/Ca ratio varies as a function of the/Ö2, ranging from values of about 0.2 in many CRB piemontites to values higher than 0.4 in the CRH manganian epidotes. In the silicate rocks, variations in the Mn3+/Fe3+ ratio of piemontite are thought to be mainly due to the presence of braunite and hematite which fractionate Mn3+ and Fe3+, respectively. The absence of braunite in SRH results in Mn3+/Fe3+ ratios even greater than 5 in these piemontites (Pm2o-29Ps6-17), as against 1.5 in the SRB piemontites (Pm19Ps13), despite the fact that, the whole-rock Mn3+/Fe3+ ratio is higher in SRB than in SRH. The Sr content of the piemontites in the studied rocks directly related to the total Sr content of the whole-rock.