FMF - Friends of Minerals Forum, discussion and message board

[prcantos]

I am really pleased to start this thread concerning rocks, after a similar and successful initiative at Spanish FMF Forum. I am so grateful to site admins for asking me to begin explaining what this guide consists of.

We are trying to collect photographs of rocks all around the world and as much information as possible about them: age, origin, description, composition... Rocks are closely related to minerals, as we usually find them forming rocks or growing on their surfaces and cavities. We intend to make a collective work where everybody can contribute and where expert opinions of geologists an petrologists is widely appreciated.

Photographs should show the macroscopical appearance of the specimen, though detailed pictures are usually advisable.

In order to keep the guide in order and reach and to guarantee a significant scientific level, you should proceed according to these rules:

1) If you are not sure about your rock specimen (mostly the name or the kind of rock), you should create before a new thread at "What is it? - Where is it from?" section giving the pictures and as much information as possible.

2) When the rock is properly identified, you can upload the photographs and the data in the corresponding section of FMF Rocks Guide (Igneous Rocks, Sedimentary Rocks or Metamorphic Rocks). If the rock do not fit a single group (e. g. some serpentinites, charnockites, some slates...), please choose the most accurate group in your opinion and explain it.

3) Use FMF fields at uploading photographs in this way:

*) MINERAL: write the whole name of the rock (name, varieties if known...)

*) LOCALITY: write the whole name of the location where the rocks was collected (include as many as possible: deposit, mine, geographical name, geological formation name, locality, region, country...). Please be precise at this point, your rock will be seen worldwide!

*) SIZE: in figures, or put some reference next to the specimen in the picture (a coin, a key, a pen...)

*) COMMENTS: write here the rest of the information, everything you consider to be interesting.

Of course further opinions and discussions are always welcome! Prosit!


FMF Guide of Sedimentary Rocks

Sedimentary rocks can be classified according to different criteria. A chemical classification contains these families:

[1] Siliceous and siliciclastic rocks: conglomerate, breccia, sandstone, siltstone, non-crystalline or microcrystalline forms of silica (opalo, chert, calcedonia...)

[2] Carbonate rocks: limestone, dolostone...

[3] Phosphatic rocks: phosphorus-rich rocks, e. g. phosphorite, phosphate nodules, bone beds, phosphatic mudrocks...

[4] Saline rocks: evaporites (halite, gypsum, anhidrite...)

[5] Iron-rich rocks: banded iron formations and similar.

[6] Manganese-rich rocks: nodules and layers in deep marine volcanic environments.

[7] Organich-rich rocks: varieties of coal, bituminous slates...


A genetic tipology may be:

[1] Clastic rocks: rocks containing particles which were placed in sedimentary basins, e. g. conglomerate, breccia, sandstones, lutites...

[2] Chemical sedimentary rocks: formed by inorganically precipitacion of minerals from supersaturated solutions, e. g. limestones, dolostones, evaporites...

[3] Organic sedimentary rocks: from incomplete decomposition of organic substances, e. g. coals, oil-petroleum, some bituminous rocks...


References

W. Maresch & O. Medenbach, Rocas, Barcelona (Spain) 1990.
F. Bastida, Geología, una visión moderna de las Ciencias de la Tierra (2 vols), Gijón (Spain) 2005.

[prcantos]

A sedimentary rock bearing fossils.

[prcantos]

A glacial conglomerate (tillite).

[prcantos]

Gypsum-rock from an evaporitic basin which was a Pliocen lake.

[Jordi Fabre]

Per Pablo request:

[prcantos]

Siltstone is a consolidated detritic sedimentary rock with grain size between 0.002 (clays, or lutites, or pelites) and 0.063 mm. (sand, or psammites). Quartz and clays minerals are the main components, there may be many different accesories: mica, oxides, zeolites, calcite, dolomite, gypsum... It is often associated to clays, but there is a noticeable difference: siltstone has a rough touch while clays have a soft or greasy touch.