FMF - Friends of Minerals Forum, discussion and message board

[Jordi Fabre]

Art send to us a question concerning a Quartz geode formed where a sponge fossil used to be. It is this specimen:

https://www.fabreminerals.com/search_show.php?SECTION=ES&CODE=VB16J1

the Art's question was:

I would like to have more pieces of information about it, if you can
help me out:
(2) What is the age? (late Devonian, early Cretaceous, etc.)
(3) What’s the evidence that this is a geode that formed where a sponge
fossil used to be?

and Carles Curto gently helps to me supplying this info:

Referring a book of Elorza, and Rodriguez-Lazaro:
"Quartz geodes (cauliflower-like) appear in a specific level of the Late Cretaceous (north of Burgos). Due to their anhydrite relicts and spherulitic quartz
(length-slow chalcedony) they are considered as pseudo of anhydrite
nodules, formed during early diagenesis. At the same level compact chert
nodules sporadically appear, presenting a considerable amount of sponge
spicules with fibrous texture (length-fast chalcedony). The silicification
of the anhydrite nodules took place before total compaction and cementation
of the sediment. The source of silica is principally biogenic (sponge
spicules). There is no link between the depositional environment and the
existence in itself of anhydrite nodules. Indirectly it is possible to see
the effects of an arid climate, which have not been recorded in the
lithologic column here described. In fact geodes aren't not strictly
sponges but yes its origin (as Anhydrite)."

The complete bibliographic cite is:

J. J. Elorza, and J. Rodriguez-Lazaro 1984. Late Cretaceous quartz geodes
after anhydrite from Burgos, Spain. Geological Magazine; 121(2): 107-113

Thanks Carles!

Jordi