Tag Archives: GeoPicture

#GeoPicture of the day: Amethyst

Evitachoel / Pixabay

For all its beauty, amethyst is a fairly common variety of quartz. This here isn’t even a particularly special image, it’s how amethyst looks like most of the time. What is it about this mineral that makes it so special?

Quartz itself is the second-most-abundant mineral in Earth’s crust. Amethyst is one of the more common of the quartz varieties. Amethyst usually has a color varying from a light violet to a deep purple. The color in amethyst is usually unevenly distributed in the individual crystals. Ironically, the color is given by a technical defect – the presence of traces of built into its crystal lattice. But even when the color is deep, the content of iron in the amethyst is rather low, ranging from 10 to 100 parts per million.

Usually, amethyst crystals don’t get very large, rarely reaching 30 centimeters or more. Previously, amethyst used to be considered one of the most valuable gemstones in the world, but with many discoveries in the world (especially in Brazil), its price has dropped dramatically, up to the point where you can find it at many flea markets.

But regardless of its market value, amethyst remains one of the most cherished gemstones. Its simple beauty just makes me stop and admire it for a moment.

#GeoPicture of the Week: Odyssey hits 60,000 Mars orbits

This image shows, in false color, the region around Gale Crater on Mars. It was taken by the 2001 Mars Odyssey spacecraft – but we just call it Odyssey.

Odyssey has been orbiting the Red Planet for 14 years, 5 months and 20 days, recently celebrating a whopping 60,000 orbits around the planet, taking pictures and making valuable observations in the process. It currently holds the record for the longest-surviving continually active spacecraft in orbit around a planet other than Earth.

This mosaic is made from Thermal Emission Imaging System (THEMIS) images used to identify the different minerals on the Martian surface. Pink indicates wind-blown dust, grey is the typical mixed Martian surface and purple is basaltic rocks. The blue color observed here suggests there’s something else there as well, but we don’t know what. The Curiosity Rover might investigate that area directly.

More information about the project here.

GeoPicture of the Week: Crinoid Fossils

Crinoids are marine animal (not plants) that have been around since the late Cambrian, some 500 million years ago. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults. They are very fragile and require specific conditions to be preserved, so fossils like this one are very rare indeed.

GeoPicutre of the Week: Fossilized crinoids

This picture of fossilized crinoids was taken at the Sternberg Museum of Natural History, in Hays, Kansas.

Crinoids are marine animals that are still alive today, even though their ancestors emerged during the early Cambrian, some 540 million years ago. They are echinoderms, related to starfish and sea urchins. They feed by filtering small particles of food from the sea water with their feather like arms and their tube feet are covered with a sticky mucus that traps any food that floats past.

GeoPicture of the week: Mimetite

Mimetite, whose name derives from the Greek which means “imitator” is not really like any other mineral I’ve seen. It’s basically a lead arsenate chloride mineral which forms as a secondary product in lead deposits.


It has no major uses, being a minor ore of lead, being usually gathered by collectors. It’s not used as a gemstone because of its softness. Alternative names of mimetite include arsenopyromorphite, mimetesite, and prixite.

GeoPicture of the week: Giant Dragonfly fossil


This is a Cast of an original fossil of a Meganeuridae. If you’re scared of dragonflies, brace yourself for this: these extinct insects from the Carboniferous period measured up to 70 cm. They are the largest known species of flying insect.

Controversy has prevailed as to how insects of the Carboniferous period were able to grow so large, especially considering that there is a fixed upper limit placed on insects, based on the way oxygen is diffused through the insect’s body via its tracheal breathing system.

GeoPicture of the day: Titanium


Full resolution here – it works this time, I promise.

Believe it or not, this is actually titanium, though not natural. It was obtained through a process called iodide process (or crystal bar process), unlike natural titanium, which is usually found chemically bonded in various ways found in rock ores. For more information, you should really check out this video (it’s actually a series with many other ones).

GeoPicture(s) of the week: Columnar basalt

This week, there’s not going to be a picture, but rather a series of picture, because you just can’t sum up the beauty of columnar basalts in a single picture.

Photo by Eric T Gunther.

Basalt is one of the more common extrusive igneous (volcanic) rocks. But how can these hexagonal columns take form? There’s no photoshop, no cutting, so how are these formed? The key here is contractional fractures; during the cooling of a thick lava flow, these contractional fractures appear. The faster the flow cools, the more contractional forces build up.

Photo by Frequenttraveller

While a flow can shrink in the vertical dimension without fracturing, it can’t easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. These columns are predominantly hexagonal in cross-section. The size of the columns loosely depends on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slower cooling leads to bigger columns.

GeoPicture of the week: Crocoite from Tasmania

Crocoite is a fairly rare mineral in many parts of the world, consisting of lead chromate, PbCrO4. The relative rareness comes from the way it forms: it requires an oxidation zone of lead ore bed and presence of what are called ultramafic rocks, which act as a source of chromium. Ultramafic rocks are 90% mafic minerals (dark colored, high magnesium and iron content) – the mantle is composed of such rocks.

GeoPicture of the week: Baryte on fluorite

What you are looking at is a picture of a baryte crystal growing on fluorite; the two minerals have not been cut or polished in any way, it is just the way they naturally are. Fluorite (or fluorspar) is a mineral made out of Calcium and Fluorine while Baryte is literally barium sulfate. It has an orthorombic, cubical symmetry. Absolutely stunning!