MINERALES DEL GRUPO DEL PIROCLORO EN LA CARBONATITA DE LA SIERRA DE MAZ (LA RIOJA, ARGENTINA)

AGUSTÍN KRISCAUTZKY; EDGARDO G.A. BALDO; FERNANDO COLOMBO

Tucuman

Congreso; XX Congreso Geológico Argentino; 2017

Asociación Geológica Argentina

Pyrochlore-group minerals in the carbonatite from Sierra de Maz (La Rioja, Argentina)Pyrochore-group minerals are a common accessory phase in the carbonatite dikes cropping out in Sierra de Maz (La Rioja, Argentina), as anhedral grains that usually reach 50 m long (exceptionally, an octahedral crystal 1 cm long was found). Almost all crystals show pervasive alteration; chemical modifications along borders and fissures are also widespread. Crystals are metamict due to high U contents. In the B site, Nb is dominant (up to 50.83% Nb2O5) followed either by Ti (13.77% TiO2) or Ta ( 23.83% Ta2O5). The A site is occupied by Ca, Na or U (or empty), with lesser quantities of La, Ce, Nd, Th, Pb, Sr, Ba y Fe. Most crystals can be classified as oxycalciopyrochlore. Fluorine rarely exceeds 0.4 apfu (atoms per formula unit).Alteration leads to lower analytical totals, decrease of Na and increase of Si (up to 9.87% SiO2) and (rarely) Ba.