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Fig. 1:- Gabbro and gabbro! Gabbro image from Geology is the Way.

What is Gabbro?

Ahh, Gabbro, everyone’s favourite time buddy. Well known for their easy-going and nonchalant attitude towards, well, everything, it seems fitting that their namesake is that of an equally nonchalant igneous rock.

“How can a rock be nonchalant, OWGS Team?” we hear you ask. Well, let's try and explain that whilst also giving you an introduction to the geological version of gabbro!

First and foremost, gabbro is a slow-cooling, igneous rock with a phaneritic texture. Igneous rocks are formed when molten magma (or lava, if it has reached the Earth’s surface!) cools and solidifies. Remember how rocks are made up of individual, interlocking minerals? The size of the minerals that make up the rock depends on the speed at which our igneous rock cools, with fast-cooling producing fine-grained, or aphanitic, specimens such as basalt, dacite, and basanite; and slow cooling producing coarse-grained, or phaneritic, specimens such as granite, diorite, and - you guessed it, gabbro!

Fig. 2:- Hand sample of gabbro. Note the phaneritic texture made up of the large phenocrysts of pyroxene! Source.

Gabbro itself is an intrusive igneous rock, meaning it cooled beneath the surface of the Earth before being exposed via tectonic uplift or weathering/erosion; extrusive igneous rocks are instead formed specifically when magma erupts as lava via volcanic events before cooling - forming lava flows, pyroclastics or tuff. As the name suggests, intrusive rock intrudes into country rock (a fancy term for already in-situ rock), which acts as an excellent insulator, therefore allowing plenty of time for the intruding magma to, excuse the pun, chill out and solidify at its own pace. Which we think is pretty nonchalant. For a rock, anyway.

Gabbro can form from large magma intrusions associated with volcanism or continental rifts, but it's more commonly formed by the mid-ocean ridges that generate oceanic crust (which gabbro is a major component of!). While their island is a little less dense than oceanic crust, we still think it's pretty neat how Gabbro remains associated with the sea in-game!

Fig. 3: Diagram showing the origins of various intrusive and extrusive rocks. Source.

Okay, so what makes gabbro, gabbro? There are lots of intrusive igneous rocks out there! To be deemed worthy enough of the gabbro title, our rock must be made up predominantly of plagioclase feldspar. 90% predominantly. Which you can see on the infamous QAPF diagram here. Additionally, they are a mafic rock, meaning they are rich in magnesium and iron and formed from a magma with relatively low silica content.

Fig 4. The QAPF diagram, which geologists use to classify intrusive igneous rocks based on their relative concentrations of quartz, alkali (potassium) feldspar, plagioclase feldspar, and foidolite. You can spot several gabbros clustered around the rightmost point of the diagram! (by Lina Jakaitė-Darkšė)

Gabbro actually shares the same chemical composition and mineralogy as its aphanitic twin - basalt! They are both derived from the same mafic magma, and are made up of mainly pyroxene and plagioclase feldspar, alongside hornblende and olivine, albeit in smaller amounts -  with the only difference being how fast they have cooled. You can actually see this under the microscope!

Fig. 5:- Thin section samples of gabbro (left) and basalt (right). Note that each sample contains the same minerals: plagioclase feldspar and pyroxene, but the size of the individual crystals differ between the samples. Source.

Find your own Gabbro!

Gabbro is sometimes referred to as black granite, and that's because it shares the characteristic large, visible crystals that granites and other igneous intrusive rocks have, albeit with many more mafic minerals. Pyroxenes and hornblendes appear as dark green or black elongate crystals, and seeing lots of these in your sample - alongside a near complete absence of quartz and plenty of plagioclase feldspar - is what you can use to determine if your rock is indeed a gabbro!

Looking at our lovely QAPF diagram, you may be a little confused that gabbros appear represented by light-coloured rocks, when they typically contain so many dark minerals. When using a QAPF diagram, we ignore all minerals that aren't quartz, feldspar, or foidolite. Ignoring the mafic minerals in our gabbro leaves us with a very light-coloured rock!

Except, it's also important to note that plagioclase feldspars aren't always light! If you're interested in feldspar diversity, check out our Feldspar post, where we go into several different kinds of feldspar. The plagioclase feldspars found in gabbros can also be quite dark, so instead of looking for a striking black-and-white mosaic in your rock, you may need to keep an eye out for a rock that looks a little more...grey and plain.

Fig. 6:- Two gabbros with light (left) and dark (right) plagioclase feldspar makeups. Situations like these are why it's important to never rely on colour alone for mineral identification. (Psst. Click on the links to compare each sample under a microscope! You can see that despite their visual differences, they still both contain gabbro minerals!)

Instead of colour, we need to look at striations in the crystals to identify if they're plagioclase feldspars. Striations are small, thin, parallel grooves on the surface of a crystal, appearing almost like consistent scratch marks. Turning a crystal so that light reflects off of the surface can reveal these subtle marks. If you see them on a nice big blocky mineral with a porcelain-like lustre, you're probably looking at a plagioclase, regardless of whether it's got a light or dark colouration!

Fig. 7:- Striations on a crystal surface of hackmanite. Many minerals exhibit striations, such as plagioclase feldspar, pyrite, quartz, and sphalerite, which can help differentiate them from similar-looking minerals.

having adult money means I can make adult purchases

like caps with the Outer Wilds Geological Survey logo on them, wehehehe

Fig. 1: Hornfels and hornfels!

What is Hornfels?

Hornfels isn’t exactly a rock, but a name for a subset of metamorphic rocks - or, more specifically, a subset of contact metamorphic rocks. You may recall that metamorphic rocks are igneous or sedimentary rocks that have been transformed by extreme pressure and/or heat. There are many ways in which these rocks can get exposed to such conditions, such as by being compressed by colliding tectonic plates, buried under many many layers of sediment, or being heated by magma below the surface. 

Contact metamorphics are just what they sound like - metamorphics formed by contact, specifically by close contact to magma that has intruded into the rocks. The magma heats up the rocks around it and literally bakes them into metamorphic rocks! 

Fig. 2: Contact metamorphics that formed when molten rock baked the country rock. The molten rock has cooled and formed a granite batholith.

Most hornfels form when sedimentary rocks such as sandstone, limestone, or shale undergo this baking process, though mafic igneous rocks (containing little silica) can also produce hornfels. Hornfels are very interesting metamorphics because they can be both very durable and very brittle. Since contact metamorphism is a type of metamorphism without any deformation, unlike other forms where rocks get folded or partially melted, hornfels may retain some features of its original rock! For example, most hornfels are black or dark green in colour, but banded hornfels, displaying striking bands of light and dark minerals, are hornfels that originated from sandstone that had light and dark layers.

Find your own Hornfels!

Hornfels can be difficult to find since they are typically very plain, but there are many features that can help you identify them! Hornfels are characteristically massive. This doesn’t mean that they are large rocks (though they certainly can be!) but that they lack any sort of foliation or bedding patterns. Big, blocky rocks that don’t break along neat planes are “massive”!

Hornfels are also incredibly fine-grained. The grains within hornfels are so small that it’s impossible to see them with the naked eye, so you won’t be finding any beautiful crystals in these rocks. Many hornfels contain significant amounts of biotite mica, and while you may not be able to see the individual crystals, their presence can create a velvety sheen or lustre over the rock.

Fig. 3: Hand sample of banded hornfels. You can clearly see the velvety sheen across the surface of the rock due to the presence of tiny mica crystals!

Due to their metamorphism, hornfels are also very hard, dense (heavy!), and often associated with a nearby igneous intrusive rock. Since they are so dense and fine-grained, hornfels also have the interesting quality of resonating when hit! If you tap on a hornfels with a hammer or another rock, you will hear a much higher-pitched sound than you may expect, almost like the sound of a bell! 

Fig. 4: While these aren't hornfels, this video has an excellent demonstration of what kind of "ringing" sound to listen for!

Fig. 5: These rocks, on the other hand, are a type of hornfels - found near the peak of Skiddaw in the Lake District of England, and were collected by a stone-mason due to their beautiful ringing properties. Said stonemason carefully chiseled each piece to create a lithophone - known today as the Musical Stones of Skiddaw. This lithophone can still be seen and played today in the Keswick Museum and Art Gallery!

None of these qualities alone will help you identify hornfels, so it’s important to identify as many of these characteristics as you can. Even still, there are many other metamorphic rocks that share these features - the only surefire way to confirm if your rock is a hornfels is to take a look at it under the microscope. The mineral grains of hornfels are very distinctly mosaic-like, giving the rock a spotty appearance, with roughly consistent sizes. Hornfels are pretty beautiful rocks at all scales!

Fig. 6: Image of a thin section of an andalusite-cordierite hornfels under cross-polarised light. Note the mosaic-like pattern of the crystals! Field of view = 2.5 cm. (by Andy Tindle)

Fig. 1: Feldspar and feldspar (specifically potassium feldspar in quartz)!

What is a Feldspar?

Feldspar is a highly diverse group of silicate minerals that are extremely common in Earth’s crust. Substitutions of different elements in their chemical makeup lead to a wide variety in colouration, habit, and even optical properties for these minerals! Think of feldspars as a game of Mad Libs, where the structure of the story (or chemical formula) is the same, but the words you use to fill in the blanks can change the overall story completely. 

Fig. 2: Visualization of the feldspar series of minerals. Note the variety and how much they can change in appearance with simple element substitutions! (by Lina Jakaitė-Darkšė)

The most common (and fundamental) feldspars are plagioclase feldspars, which are rich in sodium and calcium, and alkali or potassium feldspars, which are rich in - you guessed it - potassium! These feldspars typically come in white and pink colours, respectively, and the percentages of each of these feldspars in an intrusive igneous rock is actually how we classify them! All that differs between a granite, a granodiorite, and a tonalite are the relative percentages of white and pink feldspars they contain.

Fig 3. The Infamous QAPF diagram, which geologists use to classify intrusive igneous rocks based on their relative concentrations of quartz, alkali (potassium) feldspar, plagioclase feldspar, and foidolite. Coloured to match the typical appearances of each rock type. (by Lina Jakaitė-Darkšė)

There are many less common feldspars, too. In fact, your favourite mineral might just be one! Amazonite, labradorite (a favourite of Merry’s), moonstone, and sunstone are all varieties of feldspar, showcasing just how variable these minerals can be in appearance. Actually, your favourite rock might contain feldspar too! Feldspars love to form rocks, including some you may be familiar with, such as arkose, gabbro, porphyry, and gneiss! They are incredibly common in each rock type too, having sedimentary, metamorphic, and igneous representatives, though their susceptibility to weathering does make them slightly less common in sedimentary rocks.

Find your own Feldspar!

Go outside! Look at a rock! You’ve probably found feldspar! Because of how common feldspars are (the most common mineral on Earth’s surface!), unless you live somewhere with lots of limestone or basalt, there are probably lots of feldspars nearby just waiting to be discovered. 

If you find a rock that has pink minerals in it, odds are those are potassium feldspar crystals! Blocky white-grey minerals within a rock are likely plagioclase feldspars. Potassium and plagioclase feldspars can also intergrow with each other, forming a pink rock with white lines running through it known as perthite!

Fig. 4: Plagioclase feldspar (left) and potassium feldspar (right) can intergrow to create perthite (middle), which has beautiful pink and white markings!

Optical properties such as labradorescence (which makes it seem as if the rock is glowing from the inside) and adularescence (which creates a soft bluish sheen on the rock’s surface) are unique to labradorite and moonstones respectively, so if you are ever lucky enough to find a sample like that, you’ll know for sure that what you’re looking at is a feldspar.

Fig. 5: Labradorescence in a raw sample. Although it's much easier to see in a polished sample, raw samples also display this optical property, making it easy to tell if you've got your hands on a piece of labradorite!

Though they're very common, feldspars can sometimes be difficult to see in an outcrop since they co-occur with many different types of minerals and weather easily. It may take getting up close and personal with an outcrop to confirm if it contains feldspar!

Fig. 6: We've found some feldspars ourselves! The top left image shows a syenite (pink rock) intrusion into gneiss (grey rock) that Merry found. The top right shows an up-close picture of her hand sample of syenite, which contains lots of potassium feldspar and some quartz. The bottom left image (credit) shows an outcrop of Dartmoor granite. The bottom left shows an up-close look at a hand sample of this granite in Dekkii's collection, containing lots of plagioclase feldspar. Can you find these rocks in the QAPF diagram in Fig.3?

Fig. 1: Esker and an esker! (Geological esker image by Phil Camill)

What is an Esker?

Unlike many of the other Hearthians in Timber Hearth, Esker is named after a type of glaciated landform as opposed to a specific mineral or rock type and, other than Moraine, they are the only hearthian named after a process linked to glaciation! Pretty cool, huh?

Haha.

Ahem. 

Eskers are an interesting form of glacial deposit formed via ancient fluvial (river or stream) systems that would have snaked their way atop, through or even beneath the glacial ice. As with all fluvial systems, sediments such as muds, gravels and sands would be carried along by the water, and deposited as strata throughout the river over the course of its life.

Fig. 2: Image of a glacial river channel - Columbia University

Now, normally, a river system leaves behind a channel or a valley - leftover from when the river itself cut through the surrounding landscape via water erosion; and this presents itself in the geological record as a sudden dip in elevation infilled by sediments such as muds, sands and stratified gravel. Eskers are interesting because their topology is inverted! This is because of their glacial origins. Remember how we mentioned the rivers that form eskers would flow either atop, through or beneath glacial ice? Imagine the ice as the banks and bed of the river system. Well, when that ice melts, all that’s left behind of the system are those sediments deposited by the rivers which are then left as elevated ridges - hooray, we have an esker! So, the easiest ways to think of them are inverted river channels!

Fig. 3: A diagram showing how an esker forms - BBC

Find your own Esker!

Fortunately, due to eskers being entire landforms, they’re pretty easy to spot! But, you need to be in the correct geological context to ensure you have found an actual esker as opposed to a funky ridge! The easiest way to locate an esker is actually via satellite imagery, as they are easier to identify from a bird’s eye view! You’ll want to be in a previously heavily glaciated area and look out for long, winding ridges of stratified sand and gravel at an elevated level! They can stretch many hundreds of kilometers in length, and due to their relatively uniform topology, they are often likened to railway embankments!

If you want to try your hand at spotting an esker for yourself, we recommend having a look around Eskers Provincial Park or near Breiðárlón, Iceland on Google Maps for yourself if you want to try your hand and spotting some! Once you're confident in the shapes you're looking for, you can have a look elsewhere!

Fig. 4 : Satellite image of an esker near Breiðárlón, Iceland, - Google Maps

Eskers are an amazing geological landform as they provide us with a glimpse into the chilly past of our environment, showing how much of our landscape can be molded through glacial activity, whilst also providing us with vital fluvial palaeonvironmental data such as river direction and the speed at which the river was flowing!

Fig. 1:- Riebeck and riebeckite! Riebeckite image from mindat.org

What is Riebeck(ite)?

Lets get some nomenclature housekeeping sorted before we delve into the namesake of everyone’s favourite nervous archaeologist! Riebeck is named after the mineral Riebeckite, discovered by German explorer Emil Riebeck in 1888. Due to the German origin of the name, it is safe to assume that the correct pronunciation is “Ree-beck” as opposed to “Rye-beck” - although many folks, including Alex Beachum, utilise the latter for Brittle Hollow’s timid explorer, so perhaps it is a matter of preference over scientific accuracy. Didn’t Porphy mention something similar about the stars, once?

So, what is Riebeckite? Riebeckite is a rather fascinating mineral that belongs to the amphibole family. Amphiboles are characterised by their elongate, needle-like crystal structures and also include minerals such as hornblende and actinolite. Riebeckite is no exception, appearing as beautiful elongate, dark blue to black crystals out in the field, often within slow cooling igneous rocks such as alkali granites and granodiorites.

Fig. 2: Large Riebeckite crystals in a pegmatite (very coarse grained igneous rock), likely a syenite (left); fibrous Riebeckite (Crocilodite) from a metamorphosed Banded Iron Formation (BIF), note the “fluffy” fibres. Image Sources: left, right.

Interestingly, Riebeckite can appear in two distinct varieties, known as “habits”, as shown above: the aforementioned “massive” crystal form via cooling; and a much more brittle and fibrous variety specifically formed due to metamorphic activity.

This fibrous habit of Riebeckite, known as Crocidolite, is chemically identical to the more massive variety, but exhibits extreme differences in the form of its fibrous texture. There is still research being done into how these differences occur, but it is currently believed that tectonic stresses cause deformation in the crystal’s natural planes of weakness, eventually leading to structural defects that encourage the splitting of the crystal into thin needle-like strands known as fibres. The fine fibrous nature of Crocidolite actually places it firmly within the asbestos group of minerals, and it is more commonly known by its trade name of “blue asbestos”, which is unfortunately the most dangerous of the six types.

Fig. 3: The six types of asbestos; note the fibrous texture in all of them. These fibres can break into microscopic, sharp particles known as fibrils when disturbed, which can cause serious harm when inhaled. Image Source.

Find your own Riebeckite

Due to the dangers associated with asbestos exposure, we want to preface this section with a health and safety warning. We would advise anyone trying to obtain a sample of Riebeckite to aim for the more massive crystalline form that can be found within igneous rocks as opposed to a solid sample of isolated Riebeckite itself. If you do opt for an isolated sample, take care when handling the isolated mineral, as poor handling and abrasion can cause shedding of the fibrils that make up the crystal fibers. It is advised to store Riebeckite (and any other fibrous mineral, regardless of whether it is part of the asbestos group) in a safe container and to wash your hands after handling.

⚠️ We strongly advise against procuring a sample of Crocidolite unless you are confident in your knowledge of how to safely handle and store it. Many metaphysical stores sell harmful minerals without essential health and safety warnings. ⚠️

If you want to find your own little sample of Riebeckite out in the field, the best places to look are within high alkali granites - not sure what a high alkali granite looks like? We recommend you check out our post on Feldspar to help you ID some alkaline feldspars to help! Syenite would be an excellent place to start. You’re looking for dark, blade-like crystal growths, which are the amphibole minerals!

Fig. 4: Riebeckite crystals in an alkali rich pegmatite containing potassium feldspar and quartz. Note the dark colour and fibrous texture to the Riebeckite crystals. Image Source - Wikipedia.

Unfortunately, unless you have a specimen that is producing very large crystals, and you are well versed in mineralogy, you likely won’t be able to tell if you have Riebeckite present unless you are able to get your sample cut into thin section and under a light-microscope for further analysis; which is why many folks opt to hunt for Riebeckite at Rock & Mineral shows! If you can’t forage your own, store bought is fine ;;)

The fibrous habit, although rare, is much easier to identify and locate as it is very particular in regard to the environment it forms in - metamorphosed Banded Iron Formations, where the impacts of low grade metamorphism encourage splitting along the mineral planes of weakness to produce this habit’s distinct “fluffy” texture in the form of fibrils.

Fig. 5: Raw Crocilodite within a Banded Iron Formation, note the fluffy texture of the individual fibres. Source.

You may actually already own a descendant of Riebeckite in the form of jewellery! Ever heard of a Tiger’s Eye? Tiger’s Eye, specifically Blue Tiger’s Eye, are actually altered forms of Crocidolite, where some minerals have been changed through a process known as pseudomorphism. We won’t get into the technicalities, unless you want to ( feel free to shoot us an ask ;;) ), but the original fibrous nature of the Crocilodite is what gives Tiger’s Eye that shimmery reflective effect, known as chatoyance! This is due to light reflecting off each fibre individually, which causes a silky and shimmery lustre. Polished Tiger’s Eye jewellery itself isn’t dangerous, but it’s important to take care when handling or working with the raw form.

Fig. 7: Raw and polished “Blue Tiger’s Eye”. Note the remains of the fibrous structures in the polished version which cause the signature chatoyance Tiger’s Eyes is famous for! Please use caution when handling raw samples, as abrasion of the raw mineral can release the harmful fibrils and rock dust into the air. Always wash your hands after handling, and keep raw samples in a sealed container when not being handled. Image Sources.

Fig. 1: Our favourite flight instructor and their rusty namesake!

What is a Gossan?

Unlike the majority of Hearthians, Gossan’s namesake isn’t a specific kind of rock or mineral, but a classification of very weathered, deteriorated, oxidized rock that is rich in iron. When sulphides - minerals containing negatively charged sulphur atoms, which are often bonded to economically important metals such as copper, lead, or zinc - are exposed to the surface, they can oxidize, creating rocks with a “rusty” reddish-brown to yellow-orange appearance. These rocks are known as gossans, and are very important in mining exploration as they can telegraph the presence of an ore body beneath the surface! 

Not only can gossans indicate the presence of an ore body below them, but they can actually produce higher quality ore than would otherwise be found! As gossans weather on the surface, metals are “leached”, or freed, from the rock, moving downwards into the ground. These can produce layers of high grade metals between the gossans and their primary ore bodies in a location known as the "enrichment zone". Skilled geologists can even tell what minerals may lie beneath exposed gossans based on colouration and the subtle differences in how the rocks have weathered.

Fig. 2: A leaching diagram of gossan. When rainfall occurs, water can carry metals from the oxidized gossan downwards. These collect at the water table line and enrich the original ore with precious metals.

Find your own Gossan!

If you have a sample of malachite or azurite on a reddish-brown rock - Congratulations! You already have your very own gossan! Both malachite and azurite form by the oxidation and weathering of copper minerals, and frequently occur in gossans, creating rocks with striking red, deep blue, and emerald green hues.

Fig. 3: Oxidized copper minerals azurite (dark blue) and malachite (light blue/green) on gossan.

If you don’t have one, gossans can be pretty easy to spot out in the field due to their distinct red colouration and crumbly, rusty appearance. As weathered rocks, they’re likely to be exposed and clearly visible in outcrops. Laterite is a rock/soil type that may be confused for gossans at first glance - like gossans, it is the result of intense weathering and appears a rusty red, however it will soften and develop a clay-like consistency when wet. Gossans are a rock that will not soften when wet, and often have a characteristic spongy/porous texture. Unfortunately, not all gossans are associated with large ore bodies, so don’t count on getting rich off of finding a sample of these rusty rocks!

Fig. 4: Gossan (left) and laterite (right). Both appear as exposed outcrops with vibrant red colouration, but can be distinguished based on texture - note the spongy appearance of the gossan compared to the laterite.