Отправлено: 03.04.13 06:52. Заголовок: Nucleation, growth and oxidation of framboidal pyrite associated with hydrocarbon-derived submarine

Nucleation, growth and oxidation of framboidal pyrite associated with hydrocarbon-derived submarine chimneys: lessons learned from the Gulf of Cadiz
RAUL MERINERO1,*, ROSARIO LUNAR1
, LUIS SOMOZA2, VICTOR DIAZ-DEL-RIO
3
and JESUS MARTINEZ-FRIAS4
1
Crystallography and Mineralogy Department, Complutense University of Madrid, Avda.
Complutense s/n, 28040 Spain
*Corresponding author, e-mail: rmeriner@geo.ucm.es
2
Marine Geology Division, IGME Geological Survey of Spain, c/ Rı´
os Rosas 23, 28003 Madrid, Spain
3
Oceanographic Centre of Malaga, Oceanographic Institute of Spain, Fuengirola, 29640 Malaga, Spain
4
Centro de Astrobiologı´
a, CSIC/INTA, associated to the NASA Astrobiology Institute, Crta. Ajalvir,
km.4 28850 Torrejo´
n de Ardoz, Madrid, Spain
Abstract: In order to establish the history and conditions of nucleation and growth of pyrite formed inside methane-derived
carbonate chimneys collected in the seafloor of the Gulf of Cadiz, we compared the size distributions, morphologies, textures and
geochemical compositions of framboidal and euhedral pyrite as well as iron oxyhydroxides pseudomorphs. More than 2400
framboids and crystals were measured and four types of statistical populations were identified. Framboid size (diameter) ranges
from 6 to 12.5 mm with a mean size of 10.4 mm and a maximum value of 27.9 mm, whereas euhedral crystal size (maximum
dimension) ranges from 6.3 to 9 mm with a mean size of 7.2 mm and a maximum value of 21.6 mm. Crystal size distributions (CSD)
of different populations of framboidal iron oxyhydroxides show strongly negatively skewed shapes, close to the universal steadystate CSD curve for Ostwald ripening, an unequivocal signal of closed-system growth. Euhedral crystal populations of pyrite and
iron oxyhydroxides present CSDs with lognormal or slightly negatively skewed lognormal shapes and low size variances,
indications of open-system growth. This was confirmed by simulating framboidal and euhedral crystal populations with different
growth mechanisms using the GALOPER (Growth According to the Law of Proportionate Effect) program. From CSDs shapes and
the values of a and b2, the populations of framboids are inferred to have grown initially in an open system (surface and then
transport-controlled) and then in a closed system (Ostwald ripening) and the populations of euhedral crystals only in an open
system. In this case, the initial stage of surface-controlled growth is shorter than that deduced for the framboidal populations. We
suggest that: (1) the concentration and availability of reactive iron and hydrogen sulphide are the main factors controlling the
morphology and growth of pyrite, and (2) the higher reactive surface of framboidal textures would govern their preferential
oxidation, dissolution and recrystallization during the closed-system growth and consequently the development of higher sizes in
comparison with euhedral forms.
Key-words: framboids, pyrite, iron oxyhydroxides, crystal growth, chimneys, methane, carbonates, Gulf of Cadiz.


Eur. J. Mineral. 2009, 21, 947–961 Published online August 2009