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 Igneous Rocks

In this section you can share your igneous rocks specimen. According to IUGS - Subcommission on the Systematisc of Igneous Rocks, we consider the following families (cf. R. W. Le Maitre, ed., Igneous Rocks, A Classification and Glossary of Terms, Cambridge, UK, 2004):

[1] Pyroclastic rocks and Tepra:
Pyroclasts: fragments generated by disruption as a direct result of volcanic action, excluding rocks formed by the autobrecciation of lava flows.
Piroclastic rocks: pyroclastic deposits when predominantly consolidated.
Tefra: pyroclastic deposits when predominantly unconsolidated.

[2] Carbonatites: igneous rocks containing more than 50% modal carbonates.

[3] Melilite-bearing rocks (melilitolites): igneous rocks containing more than 10% modal melilite and, if feldspathoids ar present, melilite > feldspathoid.

[4] Kalsilite-bearing rocks.

[5] Kimberlites: nowadays we have no a single definition for these rocks, but some characterizations involving certain mineral assemblages.

Group I Kimberlites: volatile-rich (dominantly CO2) potassic ultrabassic rocks commonly exhibiting a distinctive inequigranular texture resulting from the presence of macrocrysts and, in some cases, megacrysts set in a fine-grained matrix. Mineral assemblage includes olivine, magnesian ilmenite, pyrope, diopside (sometimes subcalcic), phlogopite, enstatite and Ti-poor chromite.

Group II Kimberlites (or orangeites): ultrapotassic peralkaline volatile-rich (dominantly H2O) rocks, characterized by phlogopite macrocrysts and microphenocrysts, together with groundmass micas which vary in composition from "tetraferriphlogopite" to phlogopite.

[6] Lamproites: rocks in small dykes or smal extrussions, defined according to both mineralogical and geochemical criteria.

[7] Leucite-bearing rocks: classified according to the volcanic QAPF diagram with the prefix "leucite" or "leucite-bearing" as appropriate.

[8] Lamprophyres: diverse group of rocks that chemically cannot be separated easily from other igneous rocks.

[9] Charnockitic rocks: rocks belonging to charnockitic suite of rocks, which is characterized by the presence of orthopyroxene (or fayalite plus quartz) and, in many of the rocks, perthite, mesoperthite or antiperthite.

[10] Plutonic rocks: rocks formed by slow cooling of magma with a relatively coarse-grained ( > 3 mm.) texture in which the individual crystals can easily be seen wiht the naked eye. Use plutonic-QAPF and associated diagrams for classification.

[11] Volcanic rocks: rocks associated with volcanism whith a relatively fine-grained texture in which most of the individual crystals cannot be seen with the naked eye. Use volcanic-QAPF diagram if mineral mode is determined, or TAS classification if mineral mode cannot be determined but a chemical analysis is available.

Amateur collectors can use some simplified versions of these families, of course.


References

W. Maresch & O. Medenbach, Rocas, Barcelona (Spain) 1990.
R. W. Le Maitre (ed.), Igneous Rocks, A Classification and Glossary of Terms, Cambridge (UK) 2004.

[prcantos]

My first contribution: peridotite from Ronda (Spain).

[Pete Modreski]

Ah, a fine classic locality, the Ronda peridotite! Nice to see the rocks.
I don't know if these Rock Guides are going to generate quite the interest and interchange that the pages of fine mineral specimens do, but... it will be nice to have them!

Pete

[prcantos]

A beautiful syenite with 'labradorescence'.

[prcantos]

Andesite is an intermediate volcanic rock lying in 9 & 10 QAPF fields (if modal classification is available). Differences between basalt and andesite (they share these fields) are:

- Andesite is more silika-rich than basalt.
- Andesite has higher colour index than basalt.

Andesites are Island Arc-type and orogenic subduction zones rocks.

[mihailovici79]

Thank you for your posts. Useful explanations.

Best regards

Mihail

[prcantos]

Feldspar-rich pegmatite from central Iberian massif.

[prcantos]

Volcanic rock dacite bearing almandine xenoliths.

[prcantos]

Troctolite is a plutonic rock belonging to gabbro group, composed mainly by plagioclase feldspar (anorthite) and olivine, with small amounts of piroxene and no-quartz.

[prcantos]

A kind spanish FMF-Forum member has given permission to show here his photographs of rocks. This is really a great contribution.

I show two ingimbrites, pyroclastic rocks from Islas Canarias. Ignimbrites are a type of welded-tuff consisting of crystal and rocks fragments in a matrix of glass shards called "flames". Sometimes these components give the rock a fluent texture similar to schistosity; or maybe compactation is so developed that flames-texture is lost.

[prcantos]

I am using the title of the 1974 Amstutz collection of papers to show here some lightly altered greenish igneous rocks. I'm afraid there is no agreement on nomenclature and classifications; anyway I give here the definition by IUGS-SSIR & SSMR.

SPILITE: A term originally used for altered phenocryst-poor or aphyric basaltic rocks but later used for altered (often albitized) basaltic lavas. Now it is used for altered basic to intermediate, volcanic or subvolcanic rock in wich the feldspar is partially or completely composed by chlorite, calcite, quartz, epidote, prehnite and low-temperature hydrous crystallization products. Preservation of eruptive (volcanic and subvolcanic) features is an important characteristic of spilites. The term may be replaced by metabasalt or meta-andesite, as appropriate, regardless of the origin of the rock.

It has been widely discussed if spilites are rocks formed from a primary spilitic magma, or rocks originated by autohydrothermal or autometamorphic processes, or they are secondary metamorphic rocks.

References

* G. C. Amstutz (ed.), Spilites and Spilitic Rocks, Berlin 1974.
* R. W. Le Maitre, Igneous Rocks, A Classification and Glossary of Terms, Cambridge (UK) 2004.
* D. Fettes & J. Desmons (ed.), Metamorphic Rocks, A Classification and Glossary of Terms, Cambridge (UK) 2007.

[prcantos]

Alkaline igneous rocks are those which contain either
1) modal foids and/or alkali amphiboles or pyroxenes
or
2) normative foids or acmite (aegirine).

They come from K and Na-rich silica-unsaturated magmas.

Definitions from R. W. Le Maitre, Igneous Rocks, A Classification and Glossary of Terms, Cambridge (UK) 2004.

[Roger Warin]

Hi rock Lovers.
This crystal is a calcium-rich pyroxene, in fact augite (fassaite variety). It is not very big, but it is well formed.
Its characteristic is to have crystallized in space on an asteroid.
Indeed, the meteorite found in Argentina in 1979 at D'Orbigny is an angrite. It is a type of basaltic igneous rock, but D'Orbigny contains vacuoles that allow the crystallization of pyroxene.
The mineralogy of celestial rocks is similar to that of terrestrial rocks, but sometimes with differences. So, in meteorites there is no amphibole (which requires the presence of water).
Roger

[prcantos]

Limburgite is the non-systematic name for 'a basic volcanic rock containing phenocrysts of pyroxene, olivine and opaque in a glassy groundmass containing the same minerals. No feldspars are present' (cf. R. W. Le Maitre (ed.), Igneous Rocks, A Classification and Glossary of Terms, Cambridge 2004, p. 104). The systematic name may be hyalo-nepheline basanite. See some detailed pictures of the minerals in this rock here.