Graphite and pyrite often occur together. When they are together in a rock, they suggest that the rock was originally sediment that contained lots of organic matter.

Here is a sample of a schist that is composed of graphite (black) with layers of pyrite (greenish yellow) crystals. But where is the organic matter? It is all gone because of intense heat and pressure that followed when the rock got buried deeper and deeper. The organic matter can not, of course, simply vanish (well, it can if it transforms into oil and gas and escapes the rock). It has to transform into something else. This something is mineral graphite which is composed of pure carbon.

http://picasaweb.google.com/107509377372007544953/Rocks#5805070937587724802
Graphite schist with pyrite from Austria. Width of sample 12 cm.

But what about pyrite? Why is it there and how does it suggest that the rock originally contained organic matter? Pyrite is composed of iron and sulfur. Most commonly we find iron in combination with oxygen (minerals magnetite and hematite are very common). However, if there is not enough oxygen present, then we have a chemical environment that favors the formation of pyrite instead. What happened to the oxygen? It was probably the organic matter that consumed it when it decomposed. This is why I said that pyrite also suggests that perhaps this rock once contained organic molecules. There had to be lots of organic matter because some of it did not just decompose, it changed into kerogene (a precursor of crude oil) which later transformed into pure carbon. Graphite and pyrite occur together because they both need reducing environment to form¹.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.

Geologists have to know lots of terms to understand each other. It is especially true if we want to understand our predecessors notes because terminology tends to change as time goes by.

Today I want to demonstrate a really interesting rock that is probably composed of calcite, hessonite, and fassaite. Calcite is familiar to everyone but what were the other ones? I knew hessonite or essonite before but I had to check what fassaite means. Hessonite is a brownish variety of grossular (one of garnet group minerals, I’ve written about almandine and pyrope before) and fassaite is a light-colored greenish variety of augite (monoclinic pyroxene).

http://picasaweb.google.com/107509377372007544953/Rocks#5791275366523286002
The rock sample is composed of calcite (blue), garnet (brown grossular), and pyroxene (green augite). Mount Monzoni, Northern Italy. Width of hand sample 6 cm.

The original description says it is: “fassaite, hessonite”. I try to broaden this laconic identification hint. The blue mineral is most likely calcite because it reacts vigorously with dilute hydrochloric acid. The mineral assemblage strongly suggests that this rock sample is a calc-silicate rock. These are metamorphic rocks that contain lots of calcium-bearing silicate minerals. There are several possible ways how such rocks may form.

Perhaps it was originally impure limestone bed (that’s why we have so much calcite) that got buried deep enough for the metamorphic reactions to begin. Or maybe the limestone bed was intruded by silicate fluids from a nearby igneous intrusion. Calcite reacts easily with these fluids and the result is the formation of somewhat unusual Ca-bearing minerals like grossular, andradite, diopside, wollastonite, tremolite, etc. The rocks with such mineralogy are called skarns or tactites. They often contain economically important ore minerals as well. The most common garnet in skarn is andradite, less commonly grossular. Skarns are very interesting rocks and useful as well but their identification is somewhat complicated. There is really no easily recognizable appearance. It is a mineral assemblage (lots of Ca-bearing silicates ± ore minerals ± carbonates) that should get your attention.

In particular case it seems to me that there was magmatic intrusion involved. By the way, this locality (Mt Monzoni) gave name to the rock type “monzonite” which is an igneous rock. I am thinking that perhaps this rock formed in the contact aureole around magma intrusion (possibly monzonitic in composition although I know that in an ironic way rocks that give a name to a rock type are in the end often found to be something else) which was surrounded by carbonate rocks. This part of the Alps is composed of carbonate rocks. Even their name, The Dolomites, refers to that.

Peridotite xenoliths are solid rocks from the mantle. They are carried to the surface by magma which usually has very different composition. I wrote about dunite xenolith few days ago. It seems that these rocks are not that rare as one would imagine. I discovered some nice samples from a local geology museum.

Today I want to demonstrate you one of them from the Eifel volcanic field in Germany. It is my first video attempt. It seems to me now that I am talking too quietly but I hope you can hear something.

Agate is a rock type admired by many because of its beauty. Much smaller but still numerous and perhaps more knowledgeable group of people like breccias. Today I want to demonstrate something that should please both of them — agate breccia.

I have no idea what brecciated the rock but I think it is just beautiful.

Few explaining words about the terms used in the title:

Agate is a rock type that is composed of microcrystalline banded quartz and chalcedony. You might say that chalcedony is just a microcrystalline quartz but it is somewhat more complicated. Chalcedony is actually a fibrous intergrowth of quartz and moganite. Hence, it could be considered to be a rock type that is composed of two minerals. The compositions of chalcedony and quartz are still the same: SiO₂

Breccia is a coarse-grained rock composed of angular broken fragments of preexisting rocks. Breccias may have very diverse origins.

Dunite is an ultramafic plutonic rock that is composed almost exclusively of olivine. “Ultramafic” means that mafic minerals form more than 90% on the rocks composition. Most common mafic minerals in ultramafic rocks are definitely pyroxenes and olivine (if hornblende is present it is added to pyroxenes). Rocks that contain more than 40% olivine are peridotites. Note that this 40% means 40% of olivine-pyroxene(hornblende) pair, all other minerals are excluded in current classification scheme. Peridotite that contains more than 90% olivine have a special name, they are called dunite (named in 1864 after Dun mountain in New Zealand).

Dunite xenolith in basaltic lava from Hawaii. The sample is 8 cm in width.

‘Plutonic’ means that the rock is not volcanic, it didn’t form at or near the surface. In the case of dunite the formation place was probably very deep in the mantle. That’s why it is so rare on the surface. This rock type is rare, but it is pretty. However, its beauty is not the reason to reserve a separate rock name for it. It is an important rock type because it is probably very common in the mantle.

Dunite is mostly composed of olivine which is a bright green mineral. Fresh rock is green as well. However, olivine readily alters and loses its bright green color pretty quickly. Chances are very high that on the way up in the crust olivine grains lost some of its brightness. Hence, many samples look dull yellow, not green anymore. This rock usually contains chromite (Mg-bearing spinel group mineral). However, if the most common spinel mineral is magnetite, dunite is named olivinite instead.

Magnesium is a very common chemical element in the mantle. So we should expect to see lots of unusual mineral varieties in dunite. I already mentioned chromite, but Mg-bearing garnet pyrope is quite common as well. If the sample contains significant amount of garnet, then it should be added to its name.

Breccia is a clastic rock composed of angular rock fragments which are held together by a cement. There are several ways how breccias form. One of them (quite rare) are impact events. Meteorite explosion vaporizes lots of material but it also shatters the bedrock that is far enough to escape vaporization but is near enough to be severely hit. Such rocks are full of fractures but they still stay together and as time goes by they become one integral rock again.

The rock below is an impact breccia from Germany. The brecciation took place 14 million years ago during the impact event that created the Nördlinger-Ries meteorite crater. The clasts are composed of silicified pieces of former carbonate rock, possibly limestone. They are held together by a silica cement.

I have written about the Ries crater and its rocks before: Houses built from diamonds and impact breccia

Overview and images of gneiss as a rock type are here: GNEISS

I recently posted a picture of migmatite from Norway. Today I am itching to post another one. These rocks are really beautiful and fortunately very common in certain areas and versatile as well.

Where can we expect to find such rocks? They form deep in the crust where rocks are ductile because of enormous pressure and high temperature. Such conditions prevail under the forming mountain ranges where crust is thicker than usually. Mountains eventually erode away and rocks that were formed deep in the crust become exposed. So, if we see migmatite or gneiss, we can pretty safely say that big mountains once stood here. Migmatites are usually very old because it takes time to completely wear away mountains. Norway is a mountainous country but this migmatite didn’t form during the Caledonian orogeny (approximately 400…500 Ma) which is mostly responsible for the mountainous terrain in Norway. These mountains are still there and we have to wait few hundred million years more to see the migmatites that formed underneath them. The migmatite or migmatized gneiss I am presenting today formed one billion years before. Its age could be anywhere between 1.8…1.5 billion years. It is part of a Norwegian basement in western part of the country which is not covered by allochthonous rocks.

Article about the hydrothermal processes went online without any illustrations. It is not a paradigm shift of this blog. I still believe in photos and will continue to use them. I just didn’t have anything appropriate at the moment.

But now I have a nice photo of hydrothermally altered granite from Norway. There is a green vein in granite. One could argue that it is some sort of dike or something similar but take a closer look. There are some unaltered red feldspars and of course quartz (gray) which is very resistant mineral to all sort of changes. Dark minerals are the most reactive and they are mostly gone.

It is obvious that the green vein was granite but its mineralogy somehow changed. It is mineral epidote that gives greenish color to the vein. Epidote is a common mineral in hydrthermally altered rocks. It is quite common in granite. Such epidotized granites are sometimes named unakite and valued as semi-precious gemstones. Why is this vein in granite? Perhaps because there were some sort of weakness, most likely microscopic crack(s) that allowed hydrothermal fluids to infiltrate the rock.