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 Metamorphic Rocks

Nomenclature and classification of metamorphic rocks involves textural, mineralogical and genetical criteria, as well as widely used common names. Thus a classification of metamorphites cannot be simple. Following works and concepts by Eskola (facies), Winkler (grade of metamorphism) and Miyashiro (P/T ratio and series of facies), we can use this system:

(A) Metamorphic facies in contact metamorphism (ordered by increasing grade):

[A1] Albite-epidote-hornfels facies
[A2] Hornblende-hornfels facies
[A3] Pyroxene-hornfels facies
[A4] Sanidinite facies (very rare conditions)

(B) Low and medium P/T ratio regional metamorphism:

*) Facies at very low-grade metamorphism:
[B1] Zeolite facies (laumontite or heulandite-bearing rocks)
[B2] Prehnite-pumpellyte facies (prehnite-pumpellyte-bearing rocks, metapyroclastic rocks)
*) Facies at low and medium-grade metamorphism:
[B3] Greenschist facies (slates, phyllites, chloritoid-schists, chlorite-schists, micaschists)
[B4] Amphibolite facies (epidote-amphibolites often have been considered a transitional facies or subfacies between greenschists and amphibolites)
*) Facies at high-grade metamorphism:
[B5] Granulite facies (gneisses, granulites, charnockites, migmatites)

(C) High P/T ratio regional metamorphism (ordered by grade)

[C1] Lawsonite-albite-chlorite facies
[C2] Blueschist facies (schists bearing lawsonite, riebeckite, crossite...)
[C3] Eclogite facies (jadeite-bearing rocks, eclogites)

However, sometimes may be useful a simple system that pay attention to the composition of rocks and separate some common types of metamorphites that don't fit single facies. I propose these families according to the protolith (1-7) or the main factor of metamorphism (8-9):

[1] Metacarbonates: marble in many differents compositions and varieties, except metasomatic ones.

[2] Metapelites: rocks resulting of regional metamorphisms of clays, e. g. slates, phyllites, schists, paragneisses...)

[3] Metapsammites: rocks resulting of regional metamorphism of coarse-grained clastic sediments, e. g. quartzites, metaconglomerates...

[4] Metagranitoids: products of metamorphism of acid plutonic rocks, e. g. orthogneisses, granulites...

[5] Metabasites: metamorphites whose protolith is a basic or ultrabasic igneous rock, as serpentinites, epidote-amphibolites, orthoamphibolites, eclogites...

[6] Migmatites: metamorphites where partial melting traces are detected, e. g. migmatites, anatexites, diatexites...

[7] Metapyroclastic rocks: products of (very) low-grade metamorphism of pyroclastic rocks.


[8.1] High-pressure metamorphites: blueschists, eclogites, jadeite-bearing rocks...

[8.2] Fault-rocks: cataclasites (including tectonic breccias and fault gouge), mylonites, blastomylonites, pseudotachylites...

[8.3] Impactites: shocked rocks, impact melt-rocks and breccias (including suevite), tektites...


[9.1] Contact metamorphic rocks: spotted slates, hornfels, buchites...

[9.2] Metasomatic rocks: argillisite, greisen, beresite, gumbeite, propylite, aceite, skarn, fenite, Ca/Mg-silicate marbles...


References

H. G. F. Winkler, Petrogenesis of Metamorphic Rocks, New York (USA) 1973.
W. Maresch & O. Medenbach, Rocas, Barcelona (Spain) 1990.
A. Miyashiro, Metamorphic Petrology, New York (USA) 1994.
D. Fettes & J. Desmons (ed.), Metamorphic Rocks, A Classification and Glossary of Terms, Cambridge (UK) 2011.

[prcantos]

This eclogite comes from the Tetys Ocean Floor!

[prcantos]

Another metabasite.

[prcantos]

A three-colour mica schist from Sierra de Baza (Nevado-Filábride complex from Baetic Cordillera).

[prcantos]

Nevado-filábride marble from Baetic Cordillera.

[prcantos]

Sillimanite schist: a high-degree member of schists group.

[prcantos]

Garnet serpentinite showing deep red pyrope crystals.

[prcantos]

Two samples of metasomatic rocks.

[prcantos]

Chloritoid schist.

[prcantos]

Staurolite micaschist from Kola.

[Larry Laskowski]

Wonderful photos, great information. Very helpful for a casual but a 50 year collector of rock and minerals. As I am particularly fascinated with rocks. This site in general is outstanding and has re-sparked my interested in my life long love of collecting. Thank you all.

[prcantos]

Nice to meet you and your interest in rocks.

[prcantos]

Tourmaline may be an accessory mineral in metapelites.

[prcantos]

Some eclogites.

[Mark Ost]

I will send a picture of a sample I have . I think it is in the office so may have to wait until Monday

[prcantos]

Some non-hornfelsic contact metamorphic rocks.

[prcantos]

A blueschist from the high-pressure metamorphic terrains at the Western Alps.

Red spessartine garnets are surrounded by blue glaucophane, silver mica and white quartz. Green crystals might be omphacite or better kyanite (read the explanation at the 4th & 5th picture).