Intro to my California trip

June 12, 2025April 16, 2013 by Siim

My monthlong trip to Hawaii and California is over. I saw lots of beautiful places, covered thousands of miles and took many photos which I am eager to share with you. This post serves as an introduction to my twelve days in California.

There is little doubt that it was the best geotrip I’ve had so far. I saw many classic examples of geology which are usually shown in geology textbooks like pahoehoe from Hawaii, debris flow fans in Death Valley, and tufa towers of Mono Lake to name just a few. I saw everything I wanted to see in Hawaii (erupting volcano, green sand beach, mantle xenoliths, etc.) and in California I actually saw more than I planned. I did not believe when planning the trip that I have time to take a look at the largest trees in the world in Sequoia NP and see the beautiful landscape of Yosemite but I managed to press these into my plans as well. It is needless to say that twelve days is not enough to get to know California and its geology well but I got a taste of it and really liked it very much. So much that I definitely plan to return in the future. I will return to USA already within this year but it will be a different trip to a different place. I will write about that later when the time is right.

http://picasaweb.google.com/107509377372007544953/California02#5867496428932872882
Red Rock Canyon State Park was my first destination. It was conveniently on the road from LA to the Owens Valley and well worth a visit. It has many nice outcrops of sandstone and mudstone.
http://picasaweb.google.com/107509377372007544953/California02#5867496534395740962
It was very variable trip and not only geologically. I experienced freezing cold high in Sierra Nevada, dry heat in Death Valley and strong winds in many places. This particular morning and especially the preceding night near Obsidian Dome were quite cold. It is no wonder because this place is about 2,500 meters above sea level. I discovered that night that my old sleeping bag is no longer an adequate protection from temperatures like that. I believe it was -10 °C and I couldn’t sleep after about midnight because of cold. So I was up as soon as it was light enough. This photo was taken during the early morning hours. You can see a nice boulder of obsidian with gray pumice. Road to this place was still blocked with snow. So I had to take a 6 km hike but did so happily because it was a great way to get some warmth.

Mono Lake and many interesting geological features around it was one of the highlights of my trip. Here you can see tufa towers which grew in the lake when the water level was higher. These limestone (tufa is a type of soft limestone) towers form because the lake water is rich in carbonate ions which combine with calcium brought into the lake by springwater.

http://picasaweb.google.com/107509377372007544953/California02#5867496987590425842
American roads tend to be often straight and usually surprisingly wide. This is not something I am used to see because roads in Europe are much more winding and generally narrower also. This road descends to Death Valley from northern direction. I visited Eureka Dunes before and was on the way to see the rest of the valley.
http://picasaweb.google.com/107509377372007544953/California02#5867496986905606818
This place is near Racetrack Playa with its sliding rocks. I believe that the name “Teakettle Junction” is older and the tradition to hang teakettles there came later but who knows. Nice tradition anyway.
http://picasaweb.google.com/107509377372007544953/California02#5867497002039417346
Sand dunes near Stovepipe Wells right before sunset.
http://picasaweb.google.com/107509377372007544953/California02#5867497010590580706
Oh, these American roads. They can be a lot of fun. I can understand now why so many Americans prefer to drive 4×4 vehicles. It is a necessity in many cases. I am not wealthy enough to afford to rent these. So I have to be careful when choosing which roads to drive with a smaller car. I had no plan to go and see the Lippincott Pass but I did take a trip over the Red Pass in another range which was theoretically driveable with a 2WD car but in reality was only very barely so.
http://picasaweb.google.com/107509377372007544953/California02#5867497008540006930
I don’t know exactly what it symbolizes. Perhaps solitude and desolation because it is in one of the ghost towns. This ghost town is named Rhyolite and it is located in southern Nevada. So in addition to Hawaii and California I also visited Nevada although very briefly. I was drawn into this town partly because of its name (rhyolite is a rock type) and mostly because I just wanted to see how a ghost town in America looks like. I have to say that what I found there was disappointing. Perhaps I had an illusion of something western-like but this town seems to be younger and it has just some ruined houses. Nothing very special except this wooden structure which was the funniest thing I saw in Rhyolite.
http://picasaweb.google.com/107509377372007544953/California02#5867497039413224818
I spent most of my time in California behind the mountain ranges which makes climate very dry there. But during the last days of my trip I also saw much more populous areas of California which have more human-friendly climate. On this picture is an orange tree growing in Central Valley near the Sierra Nevada mountain range. To me it is strange to see a tree with blossoms which also carries ripe fruits. Maybe someone can explain this to me.
http://picasaweb.google.com/107509377372007544953/California02#5867497044249579026
Nice to see that some old cars are still moving. I would not choose this one as my four-wheeled companion on a geotrip but it is a delight to see those on the road.
http://picasaweb.google.com/107509377372007544953/California02#5867497049453877778
Sierra Nevada is mostly composed of granitoid igneous batholiths. This one is probably granodiorite and it hosts a nice inclusion of diorite (10 cm in length). Such inclusions are really abundant. I found them in many places and on both sides of the range.

Flowing lava of Pu’u O’o

June 11, 2025April 1, 2013 by Siim

Aloha!
I just wanted to say that I am still among the living ones and having a great time. I have spent the past 10 days or so in Hawai’i (The Big Island). I saw there lots of interesting geology. I stayed at campsites overnight and had no Internet. So unfortunately I was not able to post anything there but I hope to make it up later when I am home again. But it will take some more time because I am not finished yet. Currently I am in O’ahu at a hostel where internet is available and I can start to show you my pictures taken in Hawai’i. But soon I will go to California for about ten days and during that time I am most likely just as disappeared as I’ve been during the past two weeks. I am not even sure that there is a mobile phone coverage in areas I plan to visit like Death Valley and several other interesting but remote places.

One of the reasons to visit Hawai’i was to see living, moving and glowing lava up-close. Hawaii is probably the best place in the world to do this. I did achieve this although it was much more difficult than I imagined. Lava is currently flowing in a remote place far away from any houses or roads. To get there, I needed to hike over four miles of rough lava fields. It was really wonderful experience and I plan to write about my adventures on the lava field in a separate post when I have more time. Right now just a couple of pictures of glowing lava which comes from the Pu’u O’o vent on the southeastern flank of Kilauea. Lava is flowing there mostly in a lava tunnel but in some places it breaks onto the surface for us to enjoy the sight.

http://picasaweb.google.com/107509377372007544953/Rocks#5861729624290638754
It may be difficult to believe but it is easy to accidentally wander onto active lava flow. Its upper crust is solid. It loses heat rapidly and is only warm to the touch. It insulates the red hot interior well from the cooling atmosphere.
http://picasaweb.google.com/107509377372007544953/Rocks#5861729631102422274
In some places flowing viscous mass of basaltic melt breaks free and flows as a thick tongue of lava.

Glowing pahoehoe lava flow — Note how rapidly lava loses heat and turns black. It happens about 30 seconds after flowing out onto the surface.

http://picasaweb.google.com/107509377372007544953/Rocks#5861729639091673586
Some flows were pretty impressive. This glowing river of lava is more than 5 meters wide.
http://picasaweb.google.com/107509377372007544953/Rocks#5861729650462407554
I am obviously satisfied with this. I made it although the trip was difficult and hazardous. Solidified lava flows are often hollow inside and falling through it may result in a serious injury. Active lava flows are not particularly hazardous in Hawai’i but volcanic gas that also comes out of the ground with lava is a grave threat. I knew what I was doing although at times I sensed that things are not under my control as much as I wanted. Especially because of gas. I had to search for a good place where there is lava but relatively small amount of gas for some time and always had to make sure that wind was blowing the gas away from me.

Solidifying basaltic lava flow. — This is how new pahoehoe forms. Beautiful!

http://picasaweb.google.com/107509377372007544953/Rocks#5861729655418699138
I stayed well after the sunset to admire the glowing lava field in the darkness. And then moved upwind about half a kilometer to find some place to sleep. It was impossible to hike back in the darkness because my car was about 7 kilometers away.

Here are some videos of lava flows I took that day: Videos of flowing lava and longer overview of different lava flow types.

Garnet

June 15, 2025March 13, 2013 by Siim

Garnet is a dense and hard silicate mineral which occurs in many rock types, but it is especially common in some metamorphic rocks like schist and amphibolite. It is a common rock-forming mineral in some igneous rocks.

Almandine — Crystals are usually reddish and isometric. These almandine grains are picked from a beach sand. Redondo Beach, California, USA. Width of view 10 mm.

It is hard and resistant to weathering which makes it a very frequent component of sandy sediments. Garnet is almost nowhere a dominant mineral (it is one of the principal minerals in eclogite), but it is present in variable amounts in a wide variety of rock types and sediments. It is easily noticeable because of intense and contrasting coloration (mostly red) and because it often stands out from the surface of the rocks.

Garnet is actually a mineral group, not a single mineral. These minerals share similar crystal structure, but they have a variable chemical composition. The general chemical formula is X₃Y₂(SiO₄)₃, where X cations are mostly Fe²⁺, Mn²⁺, Mg, and Ca and the Y cations are Al, Fe³⁺, and Cr³⁺.

Garnets are divided into two groups.Those with Al in Y structural site are the pyralspites and those with Ca in the X site are ugrandites. These strange names are derived from the first letters of the single minerals in these groups. Pyrope, almandine, and spessartine make up the pyralspite and uvarovite, grossular, and andradite are the members of the ugrandite group.

Here are the common members of the garnet group and their chemical composition:

Mineral	Composition	Group
PYROPE	Mg₃Al₂(SiO₄)₃	Pyralspite
ALMANDINE	Fe₃Al₂(SiO₄)₃	Pyralspite
Spessartine	Mn₃Al₂(SiO₄)₃	Pyralspite
Grossular	Ca₃Al₂(SiO₄)₃	Ugrandite
ANDRADITE	Ca₃Fe₂(SiO₄)₃	Ugrandite
Uvarovite	Ca₃Cr₂(SiO₄)₃	Ugrandite

Pure endmembers, however, are very rare. There is an extensive solid solution within pyralspite and ugrandite groups, but only limited amount of substitutions are possible between these groups. Hence the need to separate them into two groups. Specific name of a garnet group mineral depends on the dominant cation. It is an almandine if Fe²⁺ is the main cation in the X site. Almandine is the most widespread mineral of the group. Uvarovite is commonly described as a common garnet group mineral although it is rare in nature and occurs only in specific chromium-rich rocks. Possible replacements in the lattice are not restricted to those mentioned above, but these are the most important ones.

Almandine is a common mineral in metamorphic rocks that formed when buried in crust under the load of at least 10 km of rocks and sediments¹. This rock sample is a schist (metamorphosed clay-rich sediments) that contains many common Al-bearing porphyroblasts like almandine (red, equant), staurolite (dark, elongated), and kyanite (light blue, elongated) in a light-colored groundmass of muscovite (mica). Width of sample is 7 cm.

Garnet group minerals crystallize in the cubic system – they all show roughly equant dimensions (no elongation). Garnet in rocks may demonstrate beautifully developed crystal faces. They are outstandingly dense minerals for a silicate mineral with such a composition. Their specific gravities range from 3.58 (pyrope) to 4.32 (almandine). This is a result of close packing of the crystal structure which allows garnets to be common minerals deep in the crust and mantle. It is also physically hard, some garnets are even harder than quartz. This property and a lack of cleavage makes it a potentially good abrasive material and garnet is indeed frequently used for that purpose.

Rocks that host garnets are relatively good guides that help to identify the specific garnet species. Red equant grains in mica schists belong to iron-rich variety almandine. Pyrope is a Mg-rich variety that occurs in (originally) deep-seated rocks like peridotite, kimberlite, eclogite, or serpentinite. Spessartine, the manganese-rich variety of the pyralspite group, is common in magmatic rocks, especially pegmatite. Ugrandites are typical in metamorphosed calcareous rocks like skarns. This is simplified approach, of course. For example: almandine also occurs in igneous rocks, not only metamorphic rocks. And spessartine also occurs in skarns.

The color of garnet is primarily controlled by its composition. Pyralspites are either red, orange, purple, or even almost black. Grossular and andradite are yellowish brown to black or green. Uvarovite is bright green. Garnet crystals are beautiful because they are intensely colored and often have nicely developed crystal faces.

Garnet is a well-known mineral not only because it is so widespread, but mostly because of its deep red color and beautiful crystal faces which make it a semi-precious gemstone. Industrially garnet is mostly used as an abrasive because of its hardness and irregular fracture. It is also used in water purification filters.

Garnet is a common mineral in some igneous rocks. Pegmatites may contain beautiful almandine or spessartine crystals. This pegmatite is composed of spessartine, sodic plagioclase, and muscovite crystals. Width of sample 10 cm.

Colorful garnet peridotite — Purple Mg-rich pyrope is a common ingredient of ultramafic rocks from the mantle. This sample of peridotite from Åheim, Norway also contains green chromian diopside and yellow olivine. Width of view 25 cm.

Almandine from Idaho — Almandine grains from Emerald Creek, Idaho, USA. Width of view 15 mm.

Andradite (garnet) crystals — Andradite (demantoid) crystals. Andradite is not usually green, but demantoid is a green variety of andradite that owes its color to chromium that is partly in place of iron in the crystal lattice. Width of view is 30 mm.

Melanite crystal — Melanite (Ti-bearing black andradite) in an alkaline igneous rock. Note well-developed crystal faces. Kaiserstuhl, Germany. Width of the large crystal is 4 mm. The specimen belongs to the Museum of Geology of the University of Tartu.

Beach sand from Sri Lanka that contains lots of heavy minerals like almandine (pink) and spinel (dark red). Width of view 20 mm.

Garnet concentrated from a beach sand in Australia. Garnet is used as an abrasive material. Width of view 20 mm.

Garnet sand — Garnet-rich fraction of heavy minerals sorted out by running water near the coastline at Pfeiffer Beach, California.

A closer look to the Pfeiffer Beach sand. Pink mineral is almandine. Width of view 8 mm.

Almandine grains are often present in granitic igneous rocks (S-type or peraluminous granites that have a sedimentary protolith). Width of sample from Estonia is 8 cm.

Sometimes garnet crystals are very concentrated in beach sand. This sand sample is from Nome in Alaska which also contains gold. Width of view is 10 mm.

http://picasaweb.google.com/107509377372007544953/Rocks#5854164931185675298
This sand sample obviously comes from a weathered metamorphic terrane. It is composed of schistose lithic fragments, mica, garnet, and plagioclase feldspar, among others. Width of view is 20 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5791275366523286002
Grossular and andradite (Ca-garnets) are common constituents of calcareous metamorphic rocks like skarn. Skarn is a result of a reaction between magmatic hydrothermal fluids and carbonate rocks. The rock sample is composed of calcite (blue), grossular (brown), and pyroxene (green diopside). Skarns may also contain economical metal-bearing minerals. Mount Monzoni, Northern Italy. Width of sample 6 cm. TUG 1608-4882.
http://picasaweb.google.com/107509377372007544953/Coll#5853356229844711730
Calc-silicate minerals andradite (brown), diopside (green), and wollastonite (white) in a skarn. Width of view 5 cm. TUG 1608-4877.

Mica schist rock sample — Almandine is a common mineral in aluminous metamorphic rocks. This is a sample of garnet-muscovite schist (mica schist). Narvik, Norway. Width of sample 14 cm.

Caption — Almandine porphyroblasts in amphibolite from Southern Norway. Width of sample 16 cm.

http://picasaweb.google.com/107509377372007544953/2015#6196127317767358722
Small garnet porphyroblasts in amphibolite. Senja, Norway. Width of sample 11 cm.

Garnet hornblende schist. The width of the sample is 19 cm. — Garnet hornblende schist from Switzerland. Width of sample 19 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190951326487278434
Garnet with magnetite and quartz in a metamorphosed heavy mineral sand deposit. Varanger Peninsula, Barents Sea, Northern Norway. Width of sample 36 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951168417918930
Pyrope in a peridotite (wehrlite) with green chromian diopside and yellow olivine. Åheim, Norway. Width of view 25 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190953178323592850
Garnet crystals stand out because of good resistance to weathering. Olivine (yellow) has lost its original green color. Green mineral is pyroxene (diopside). Width of sample 11 cm. Åheim, Norway.

Garnet rims in an anorthositic (plagioclase-rich) coronite with ortho- and clinopyroxene. Width of sample 13 cm. Holsnøy, Norway.

http://picasaweb.google.com/107509377372007544953/2015#6191004448558737922
Another coronite (anorthositic granulite) from Holsnøy. Garnet rim is surrounding a core of orthopyroxene. White mineral is plagioclase feldspar. Width of view 36 cm.

Large crystal in an ultramafic rock peridotite. Hullvann, Norway. Width of sample 18 cm.

Lots of garnet crystals (compositionally between almandine and pyrope end-members) in a metamorphic rock eclogite. Holsnøy, Norway. Width of sample 9 cm.

*Very fresh-looking eclogite with bright green (omphacite) and red (garnet). Nordfjord, Norway. Width of view 12 cm.*

http://picasaweb.google.com/107509377372007544953/2015#6191004675414948226
Garnet in eclogite. Width of view 20 cm. Selje, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6196126895657298962
Garnets are common minerals in high-grade metamorphic rocks granulites. Associated minerals are quartz, cordierite and feldspar. Width of sample 12 cm. Tankavaara, Inari Granulite Belt, Finland.

Garnet and biotite are the main components of this unusual pegmatite. Width of sample 13 cm. Senja, Norway.

http://picasaweb.google.com/107509377372007544953/2015#6196127268251431170
Idiomorphic garnet crystals in a pegmatite with very unusual composition: garnet with biotite. Width of sample 13 cm. Senja, Norway.

References

1. Wood, B. J. (2007). Garnet. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 7. 686-687.
2. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.

Bog iron

January 11, 2016March 12, 2013 by Siim

Bog iron is a type of iron ore composed of mostly mineral goethite and other iron-bearing minerals with impurities like clay or plant debris.

http://picasaweb.google.com/107509377372007544953/Rocks#5854471542847046882
Bog iron is typically rust-colored and composed of limonite. It is a mixture of iron-bearing minerals, most important of them is goethite. Width of sample from Estonia is 14 cm.

It used to be a very important ore of iron before better alternatives like banded iron formations were discovered. It has no industrial significance anymore but not because its iron content is low. Bog iron may in many cases be quite rich in iron. The main problem is that it forms only a thin layer of rust-colored and porous layer at the bottom of bogs. It simply is not economical to search for such thin and laterally widespread deposits.

Bog iron is associated with bogs because bog water contains very little oxygen and can therefore hold iron. Oxygen would quickly react with iron and precipitate it as a rusty ferric iron (with an oxidation number of +3) which is insoluble in water. Iron with an oxidation number of +2 (ferrous iron) is soluble and can be held in solution by bogs. Bog iron forms if something provides oxygen. It can be oxygenated ground water, for example¹.

This mineral resource is associated with northern latitudes where peat bogs are widespread. Such areas are Scandinavia in Europe, and extensive areas in northern Russia and Canada.

http://picasaweb.google.com/107509377372007544953/Rocks#5854471532647116290
Bog iron is not massive. It contains lots of pores and its density is lower than one would assume from an iron ore. Width of sample from Estonia is 12 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5854471542965091826
Width of sample from Russia is 13 cm.

References

1. Robb, L. (2005). Introduction to Ore-Forming Processes. Blackwell Science Ltd.

Great circles and flight paths

June 13, 2016March 8, 2013 by Siim

It is an often repeated cliché that the easiest way to become a millionaire is to start out a billionaire then go into the airline business. It is a tough and competitive business which means that we can be fairly certain that planes do fly the shortest possible way between point A and B. Sure, there are many complicating factors but let’s leave them at the moment.

It is an amusing experience to take a globe or Google Earth and find out how an airplane should fly between two points to spend the smallest amount of fuel possible. In order to do that, an airplane should follow a great circle. What is this? It is an imaginary line on the surface of the Earth which is an intersection of the surface and a geometrical plane which is determined by three points: departure and arrival airfields and the center of the Earth.

When we open Google Earth and measure the distance between two points, this distance is measured exactly along a great circle. There is nothing very special about that if these points are relatively close to each other (few thousand kilometers) or if they are roughly on the same meridian (all meridians are great circles). However, things get somewhat surprising when these conditions are not met.

The result is surprising because our Earth is round but we prefer flat maps for practical reasons. Unfortunately, there are no ways to project a spherical surface onto flat map without generating distortions. It is a matter of choice which distortions and where we prefer but we can not go without them. We should keep this in mind or otherwise we may live in illusions which have not much to do with the real world.

This post is motivated by my upcoming trip to Hawaii. I live in Estonia which is a small country in northern Europe. Now let’s imagine how should an airplane fly in order to cover the smallest possible distance. Is it something like this:

http://picasaweb.google.com/107509377372007544953/Coll#5853054459213578562
Actually no, this is not the shortest distance between Estonia and Hawaii. We should not believe this map because it is a two-dimensional representation of a three-dimensional object. Hence, it is unavoidably faulty and limited. In reality, the shortest flight route between Estonia and Hawaii goes almost straight over the north pole:

http://picasaweb.google.com/107509377372007544953/Coll#5853054494721766546
Although there are few countries that have more northerly position than Estonia and Hawaii is not too far from the equator but still the shortest way to Hawaii from Estonia is to head straight towards the north pole. This world is sometimes really weird. Or I should say “round”?

I said in the previous post that after the trip Hawaii would be the most distant place on Earth where I have ever been. This is really nothing to brag about because Hawaii is only 11,000 kilometers away from my home. The circumference of the Earth is 40,000 kilometers. We should travel approximately 20,000 kilometers and then we can say that it really is not possible to go any further, unless we are leaving the Earth entirely.

But let’s come back to the real world. It would be very nice to fly straight from Estonia to Hawaii but this is not to be. It is odd and very unfortunate. Both are fine places for living but surprisingly small number of people are apparently sharing my view. I have to first go to places where more people live and where there is a real market for airlines. In my case, instead of heading north, I will go south. I will go to Istanbul in Turkey from where my plane should fly to Los Angeles. The distance between these two cities is also 11,000 kilometers and the flight will last 14 hours. I already feel that these are going to be frustratingly long 14 hours. Anyway, here is how the route from Istanbul to Los Angeles should theoretically look like:

http://picasaweb.google.com/107509377372007544953/Coll#5853054498486073266
I know what great circles are and how bizarre the shortest route on globe may look. However, I still find it unexpected that one should fly north of Iceland over the Greenland ice shield in order to fly along the shortest route from Istanbul to Los Angeles.

I am not sure that this is the actual route because there are other factors that should be taken into account. Jet streams, for example, are powerful winds high in the atmosphere which airlines try to use when flying in the same direction with wind and avoid when flying in the opposite direction. However, one thing is clear. The third image is much better representation of the actual flight route than the first one.

First time in America

June 11, 2025March 7, 2013 by Siim

I am happy to announce that I will soon leave home to discover new and interesting places. I have never been that far from home. I plan to visit two world-class geological destinations: Big Island of Hawaii and Death Valley in USA. I will go in the second half of this month and the whole trip will last about one month.

I will spend most of the time in Hawaii and about 10 days in southern California on the way back. This is not much. I know that California has interesting hiking trails and outcrops for more than a lifetime of travels. However, I have to try to take the best out of these days. I am sure some of you live in California or have been there and can recommend places worth visiting. I will arrive to Los Angeles and plan to rent a car to go to Death Valley and visit geologically interesting places along the way. I have no idea at the moment which places these are. You can help me to make the best selection.

I will try to make my best to take you along by posting pictures of interesting geology I will see there. I am really looking forward to this trip.

Olivine

June 13, 2025March 6, 2013 by Siim

Olivine is a very common silicate mineral that occurs mostly in dark-colored igneous rocks like peridotite and basalt. It is usually easily identifiable because of its bright green color and glassy luster.

Olivine is a common mineral in dark-colored igneous rocks because these rocks are rich in iron and magnesium (rocks rich in iron-bearing minerals tend to be either black or at least dark-colored). These chemical elements (Mg and Fe) are the essential components of olivine which has the following chemical formula: (Mg,Fe)₂SiO₄. Magnesium and iron can replace each other in all proportions. There are specific names for compositional varieties, but most of them are rarely used. Only forsterite (more than 90% of the Mg+Fe is Mg) and fayalite (similarly iron-rich endmember) are used more often. The vast majority of all the samples are forsteritic or compositionally close to it.

Olivine is a nesosilicate. It means that silica tetrahedra (which is the central building block of all silicate minerals) are surrounded from all sides by other ions. Silica tetrahedra are not in contact with each other. It implies relatively low content of silicon which is indeed the case. It is a silicate mineral that uses silicon very conservatively. On the other end of the spectrum is mineral quartz which is pure silica (SiO₂) without any other constituents. Other well-known nesosilicates are garnet, zircon, topaz, kyanite, etc.

Silicate minerals that crystallize from magma have a higher melting/crystallization temperature if the content of silica is lower and the content of Mg+Fe is higher. Hence, olivine has a high crystallization temperature and is therefore one of the first minerals to start crystallizing from a cooling magma. It takes silica out of magma relatively conservatively, as already mentioned. So the concentration of silica rises as olivine crystals form and next silicate minerals to crystallize (which are pyroxenes) are already somewhat richer in silica. This sequential order of crystallizing silicate minerals from olivine to quartz is known as the Bowen’s reaction series after a Canadian geologist Norman Bowen who first described it. It is one of the most important concepts every geology student is taught during the petrology course.

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

Bowen’s series or order of minerals in this series (olivine -> pyroxene -> amphibole -> biotite -> K-feldspar -> muscovite -> quartz) is a really useful one to memorize and there are several properties of these minerals that generally follow the same order. Olivine and its close neighbors are darker, contain iron and magnesium, and have a high melting temperature. Quartz, muscovite and K-feldspars are generally much lighter in color and weight, they melt at lower temperatures, and they contain no iron and magnesium. Another interesting fact is that the order of susceptibility to weathering and metamorphic alteration is exactly the reverse. It is readily altered or weathered while quartz is extremely resistant to any kind of change. All other minerals in the series are somewhere in the middle. In the correct order, of course.

Important aspect that rises from this series is the explanation why certain minerals typically form assemblages while others are almost never found together. Olivine is typically with pyroxenes (in basalt, for example) and quartz + K-feldspar with micas (biotite and muscovite) is a typical composition of granite. But there are no such rock types that are composed of olivine plus quartz. Granite and similar rocks are said to be felsic (composed of feldspar and silica) and basaltic rocks are referred to as mafic rocks (magnesium + ferric).

Olivine is a common rock-forming mineral in mafic and ultramafic igneous rocks, but it also occurs in impure metamorphosed carbonate rocks (picture below). It is a very common mineral in the mantle. Some xenoliths from the mantle are almost entirely composed of this mineral. Such a rock type is known as dunite. Olivine occurs as a groundmass mineral but also as distinct phenocrysts in many basaltic rocks. These rocks need not to be basalts in the strict sense. They may be picrites, basanites, etc. but all of them may be very similar to each other as boundaries between them are arbitrary. So it is frequently impossible to say for sure before chemical analysis is made.

Olivine is very susceptible to weathering. Bright green mineral loses its appeal rapidly in the weathering environment. It becomes dull, earthy, and yellowish brown. This material is usually a mixture of clay minerals and iron hydroxide goethite and it is known as iddingsite. It also demonstrates very little resistance to hydrothermal metamorphism. Hot and chemically aggressive fluids quickly alter olivine-rich igneous rocks into metamorphic rock known as serpentinite. It is also an important constituent of many stony and mixed meteorites. Especially beautiful is pallasite. It is a mixture of iron and olivine and is thought to represent a core-mantle boundary of a disintegrated asteroid. Perhaps the core-mantle transition of our own home planet looks something like that too.

However, there is one little thing to remember. The mantle is indeed most likely compositionally close to it, but most of it is not composed of this exact mineral. Olivine tolerates well pressures in the crust and in the upper mantle, but at 350 km depth its crystal structure starts to break down. The composition remains, but it takes a new and more compact form. It is not technically olivine anymore because minerals have a definite crystal structure.

http://picasaweb.google.com/107509377372007544953/Rocks#5852183063536948050
Olivine is not just an igneous mineral. It also occurs in impure metamorphosed carbonate rocks. Here olivine crystals are found in a sample of calcitic marble. Some crystals even possess a typical crystal faces which are usually lacking in igneous rocks because olivine grains are often corroded (they reacted with the melt surrounding them). Width of sample is 9 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5841862778873146786
Phenocrysts in ultramafic picritic rock from La Palma, Canary Islands. Width of sample is 5 cm.

Olivine augite basanite — Weathered olivine is dull, earthy, and usually yellowish brown mixture of clay minerals and iron hydroxides. Black grains are pyroxene phenocrysts. Rock sample is basanite (ankaramite) from La Palma.

Dunite with dark green chlorite. Helgehornsvatnet, Norway. Width of sample 11 cm.

Basalt or picrite from Oahu with lots of slightly weathered olivine. Width of sample 6 cm.

Dunite — A sample of dunite which is composed of almost pure olivine. It is mined because of its high forsterite content. Olivine is used mostly as a refractory material. Width of sample 9 cm.

Olivine (orange weathered spots) is a major component of gabbroic rock troctolite. Gray is plagioclase. Flakstadøya, the Lofoten Archipelago, Norway. Width of sample 15 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190951168417918930
Olivine (yellow) with pyrope (purple) and chromian diopside (green) in peridotite. Åheim, Norway. Width of view 25 cm.

Serpentinite — Chrysotile is an asbestos mineral that belongs to the serpentine group of minerals. These minerals are the result of hydrothermal alteration of olivine-rich igneous rocks. Width of sample from the Sayan Mountains in Siberia is 8 cm.

Common constituents of volcanic sand. — It is a common constituent of black sand on the oceanic islands. Here are the most important constituents of one sand sample from the São Miguel Island, The Azores Archipelago. Note that olivine grains have a variable appearance (in two piles). This is the result of weathering that quickly attacks this mineral. Width of view 19 mm.

Conchoidal fracture

June 13, 2025March 5, 2013 by Siim

Conchoidal fracture is a smoothly curving fracture surface of fine-grained materials which have no planar surfaces of internal weakness or planes of separation (no cleavage). Such a curving fracture surface is characteristic of glass and other brittle materials with no crystal structure. However, conchoidal fracture is common in crystalline materials also if they have no cleavage (like mineral quartz), or if they are composed of very small mineral grains so that the fracture surface which is actually zigzagging between the grains appears smooth to our eyes. This is the case with many fine-grained (aphanitic) rocks.

http://picasaweb.google.com/107509377372007544953/Rocks#5848238745712314674
Obsidian is famous for its conchoidal fracture surface. This rock type was highly valued during the Stone Age because it makes a fine cutting blade if treated (fractured by precise and forceful blows) correctly. Width of sample is 11 cm.

Smoothly curving fracture surface develops when force is rapidly applied to brittle objects like hitting a piece of obsidian (volcanic glass) with a hard pointy object. If the force is applied correctly, a flake of obsidian is peeled away leaving obsidian with a smoothly curving fracture surface and sharp edges. It was the way our ancestors made sharp cutting tools.

Why is the fracture surface smoothly curving? Because we apply a force to only one point. This is where the brittle deformation starts. The energy of the blow spreads in the material like seismic waves travels through the Earth. So the curving lines are like the fronts of seismic energy recorded on the fracture surface. The fracture can occur only if the blow is energetic enough to peel off a flake. We need to apply more force if we want to shatter a large piece of material and much less if we just want to peel off a small flake from the edge.

http://picasaweb.google.com/107509377372007544953/Rocks#5848481665432908098
Another piece of obsidian with nicely curving conchoidal fracture surfaces. Width of sample from Armenia is 11 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5791275565177698226
This rock sample looks very much like obsidian but it is actually an amorphous fine-grained asphalt known as gilsonite. It has no crystalline structure and it is brittle at room temperature when force is applied rapidly. The sample from Mexico is 11 cm in width.

Basalt rock sample — Even crystalline rocks like basalt may display conchoidal fracture if they are fine-grained enough. However, the fracture surface is not as smooth and shiny because microscopically it is pretty rough. Width of the sample is 12 cm.

http://picasaweb.google.com/107509377372007544953/Chert#5785454442189877602
Chert is another rock type that commonly exhibits a conchoidal fracture. It was also widely used as a tool-making material by our ancestors. Width of sample from Cyprus 7 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5842989770630082594
Quartz is a mineral with crystalline structure, but there are no planes of weakness inside the crystal. Hence, it has no preferred planar surfaces along which to break. Quartz is not the only mineral without cleavage, but it is the best known and most widespread of them. Smoky quartz on the picture. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5851584483551973970
Chalcedony is a very fine-grained rock type that is compositionally close to quartz and chert. There is a red variety of chalcedony which is known as carnelian on the picture above. Red color is given by hematite impurities. The sample from Kazakhstan is 14 cm in width.

“Conchoidal fracture” is named so because the curved lines on the fracture surface resemble the rippling growth lines on the shells of clams or conchs. The sample above is sandstone with abundant phosphatic brachiopod (*Lingulata*) shells from the Ordovician of Estonia. Brachiopods look similar to clams. Their shells have similar growth ripples. Pay attention to the fact that conchoidal fracture only looks similar to the appearance of clam shells. Their origin is completely different. Width of sample is 12 cm.

Malachite

June 11, 2025March 4, 2013 by Siim

Malachite is a green copper-bearing hydrated carbonate mineral (Cu₂CO₃(OH)₂). Malachite is a minor ore of copper although it is usually used for ornamental purposes because of bright green color, interesting growth patterns and color variations. It is a well-known semi-precious gemstone.

http://picasaweb.google.com/107509377372007544953/Rocks#5851463906729299250
Malachite commonly occurs as botryoidal aggregates (having a surface of spherical shapes). Here it occurs together with limonitic precipitates. Width of sample is 12 cm.

Malachite occurs in oxidized (weathered) parts of copper ore deposits (which is usually chalcopyrite). It is the most stable copper mineral in environments in contact with the atmosphere and hydrosphere. It also occurs as a stain on fractures in outcrops, as a corrosion product of copper and its alloys, and as suspended particles in streams and in alluvial sediments¹.

There are other copper-bearing green minerals. Malachite can be distinguished from them quite easily because it is a carbonate mineral. Most carbonates, including malachite, are effervescent in dilute hydrochloric acid.

The formation of malachite starts with the dissolution of sulfide ore. Very acidic meteoric water (pH 2…3) is needed for that. Acidity is provided by the oxidation of sulfide minerals themselves, like pyrite and chalcopyrite. This results in sulfuric acid being produced in large quantities which is mixed with water and makes it very corrosive. Acidic water attacks the ore deposit and liberates copper from it. Copper-bearing meteoric water is then moving away from weathered ore and copper starts to precipitate when the acidic water encounters a neutralizer like limestone country rock or calcite vein. These materials and carbon dioxide also provide material for carbonate ions². It is stable when the pH is over 5 and it precipitates above the water table.

Dirt from a copper mine dumps in Namibia. It is a mixture of several weathering products of copper ore like malachite, chrysocolla and chalcophyllite. Width of view 20 mm.

http://picasaweb.google.com/107509377372007544953/Rocks#5851474958822797794
A mining hutch with rocks stained by malachite at Timna in Israel. Timna Valley in the Negev Desert is the location of the oldest copper mine in the world. Copper mining at Timna started approximately 6000 years ago.

References

1. Einaudi, Marco T. (2007). Malachite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 10. 364.
2. Nesse, William D. (2011). Introduction to Mineralogy, 2nd Edition. Oxford University Press.

Chrysotile

June 13, 2025March 3, 2013 by Siim

Chrysotile is a fibrous mineral that belongs to the serpentine mineral group. Other members of the group are lizardite and antigorite. All serpentine group minerals share the same chemical composition (Mg₃Si₂O₅(OH)₄) but they have different crystal structures. Chrysotile is the only one among them with a fibrous habit.

Chrysotile occurs as cross-cutting veins in serpentinized rocks. Serpentinization is a hydrothermal (temperature below 350 °C¹) metamorphic process that affects magnesium-rich igneous rocks like peridotite and pyroxenite. These are rocks that contain lots of olivine and pyroxene. These minerals are altered to serpentine group minerals plus magnetite. Magnetite forms because ultramafic igneous rocks contain lots of iron but serpentine group minerals contain no iron at all. So the iron just has to form its own phase. This is the reason why serpentine minerals often seem to be weakly magnetic. They are not but they usually contain lots of small magnetite crystals.

Most serpentinites form in the oceanic crust which is heated from below and percolated by ocean water. Obduction of such rocks accounts for serpentinites squeezed between continental blocks. Such former pieces of oceanic lithosphere which are now part of the continental lithosphere are known as ophiolite complexes.

Chrysotile aggregate is composed of many parallel long and very thin rolled tubes of chrysotile sheets. Each tube is about 20 nanometers thick. It is far too thin to be seen with the unaided eye. The fibers that we see are actually aggregates of many parallel rolled tubes. The length of the tubes is variable but may be up to several centimeters. Chrysotile forms rolled tubes because their crystal structure is composed of two layers, one of them being slightly smaller. This difference is compensated by rolling the sheet so that the shorter sheet remains inside.

Chrysotile asbestos and associated health hazards

Chrysotile is the most commonly used asbestos mineral although its golden days are clearly over because of health concerns. Asbestos minerals do not form a single mineralogical whole. Most of them are amphiboles. Chrysotile is the only non-amphibole asbestiform mineral. This is why it perhaps deserves to be treated separately from other asbestos minerals. Grouping them all together is a dubious practice, at best, that simply demonstrates a lack of scientific literacy.

The health risks associated with asbestos are very real but one should be able to make a clear difference between serpentine and amphibole groups which are most likely not equally hazardous. Another aspect that seems to be poorly understood is that pulmonary disease may develop as a result of intensive and long-term exposure to asbestos fibers in the air. Chrysotile or white asbestos as it is frequently called is likely not as hazardous substance as it is usually imagined to be.

The reason why people are so afraid of it is in my opinion their ignorance. They simply don’t know what it is — have never seen, have no personal experience. Perhaps they even don’t know that this is a natural material. The situation is somewhat similar to nuclear energy which is also very dangerous stuff if handled the wrong way. However, in my opinion it is really the only serious alternative to fossil fuels at the moment. Unfortunately, the scientifically illiterate public just do not understand what are atoms and how on earth they can be used to make electricity. They can not see nor sense radiation and are therefore frightened and don’t want to know anything more about it. I guess most people have no idea that we are all the time exposed to radiation. The same seems to be true with particulate mineral matter. Our bodies are under constant biological and mineralogical attack but we are not that easily knocked out. Our immune system and repair mechanisms are effective and can cope with most of the dangers. It is extremely unlikely that we have not developed an adequate defense against low-level exposure to mineral dust, fibrous or not.

I already mentioned fossil fuels and nuclear energy. There is one more thing that I would like to say about it. The current situation in the world proves that there really are no serious alternatives to fossil fuels. We have postponed or cancelled lots of nuclear energy projects in many countries after the Fukushima accident and as a result our appetite for shale gas, tar sand, oil shale, and coal are going straight up. I don’t want to think what it means to our climate. I have actually largely given up worrying about that because firstly, I can not do anything about it and secondly, I will be dead before the situation gets really bad. It was ironic and desperate remark. I do actually feel responsibility but to my grandchildren who might ask why did you (I mean the mankind living today) do it I simply say that I am not powerful enough to alter the course we as a human race have chosen to follow. I wrote my blog to educate people but only a handful of them read that. And those who read already knew it all. People that should read and learn are unfortunately not doing it. So my impact is almost nil anyway and I guess that blogging won’t count as an excuse, unfortunately.

Properties and uses of chrysotile

Chrysotile has been extensively used in the past because it has many useful properties. It is a good thermal and electrical insulator. It also absorbs sound and is chemically inert. Chrysotile is fire-resistant and absorbs mechanical energy. Its fibers are flexible with enough tensile strength to be woven. It is perhaps needless to say that it was once considered to be an almost ideal material for hundreds of versatile industrial applications.

http://picasaweb.google.com/107509377372007544953/Rocks#5850409755054232642
Lots of chrysotile veins in serpentinite from Tuva near Mongolia. Width of sample is 9 cm.

References

1. Baronnet, Alain J. (2007). Chrysotile rock. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 4. 151-153.