The research during the construction of the M-50 highway has allowed the study of the silica levels that
constitute the most important Neolithic flint mine in the Iberian Peninsula. A group of more than 3.800
extraction pits were realized to obtain cherts and opals that appear among Mg smectites beds (Aragonian
deposits). The description of the nodular and stratiform silica levels and the study of their types and
distribution have been done at geological sections obtained during the archaeological excavations. Three
petrological types of siliceous rocks (chert, opal and opaline chert) are established and defined according
to their mineralogy and microfacies. Cherts are constituted only by quartz and locally minor amounts of
moganite. Opals contain opal CT, Mg smectites and minor quartz. Opaline chert is defined when the
proportion of quartz is higher than 50%. All the types of siliceous rocks reproduce bioturbation structures
as well as other structures found in the host rocks (grains, intraclasts, brecciation).
The silica levels at the base of the columns are constituted only by chert and were formed by the silicification
of nodular and lenticular dolomite levels included in the Mg-smectites beds. The rest of the siliceous levels
are composed of opals and opaline cherts and were formed by silicification of Mg-smectites. The outer part
of the opaline chert has a higher content in opal and clay relicts whereas the inner part is mainly microcrystalline
quartz. This change is consequence of the recrystallization of the opal (aging) producing a more compact
silica rock. The main silicification occurred under the groundwater table because the structure of the host
rocks is conserved in the silica rocks. A general question is whether the silica that precipites is introduced by
groundwater or it is released from the dissolution of clays during their replacement
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