The Science of Everything Podcast - Episode 74: Minerals and Rocks

Episode Date: December 30, 2015

A discussion of minerals, including their main properties such as crystal structure, habit, cleavage, lustre, and hardness, followed by a brief overview of some of the major mineral classes, with a focus on the various types of silicate minerals. I also discuss the rock cycle and the relationship between the three major types of rocks: igneous, sedimentary, and metamorphic. 

The Science of Everything Podcast, episode 74, Minerals and Rocks. I'm your host, James Fodor. In this episode, we're going to take a look at rocks and minerals. So we'll start with looking at minerals. We'll define what mineral is. Look at some of the properties of different minerals. So crystal structure and habit, hardness, luster, cleavage, and things like that. And then I'll go through just some of the main mineral classes, in particular, the silicates, halites, sulfide, oxides,

some of the other main types of minerals. And then I'll move on to start talking about rocks, we'll give a brief definition of rocks. We'll look at the three main types of rocks. It can be a sedimentary and metamorphic, some of the subclassifications within each and a few of different properties and other interesting tidbits.

No particular recommended background for this episode, although some basic knowledge of chemistry. So, for example, the sort of thing that I talked about in episode eight, atoms and molecules, would be somewhat helpful for some of the discussion about minerals. Okay, so let's make a start. So first of all, starting with minerals. You've probably heard of minerals before.

You may have heard of them in the context of vitamins and minerals. That's a nutrition context. The definition of mineral there is not the same as the definition of mineral used in geology, so it's the latter that I'm going to be focusing on here. In geology, a mineral is a naturally occurring substance that has a natural-occurring substance that a specific chemical formula and a crystal structure and it's inorganic so those are the four short of tuition properties inorganic specific chemical formula naturally occurring crystalline structure however some of those have come

under some dispute or debate recently particularly that having a specific chemical formula and being completely inorganic that is not derived from biogenic processes because there are some minerals or mineral-like substances that are biogenic and there are others that have some variability in their chemical formula, so those are somewhat ambiguous requirements. Naturally occurring in crystal structure though are essential, so if something's man-made, it's not considered to be a mineral. Having a crystal structure means that the atoms are arranged in an orderly lattice-like arrangement structure. So glass, for example, is not considered to be a mineral. Well, it's generally

man-made for starters, but secondly, it does not have a naturally occurring, sorry, it does not have a regularly structured crystal arrangement of the atoms. They're sort of jumbled together in an amorphous structure. So the concept of mineral isn't necessarily extremely well-defined, but it's a useful category. I would say it's probably not something that is, sort of given to us by nature because really there's just different mineral combination, sorry, different combinations of atoms and compounds. But the study of minerals is a, is a well historically ground discipline, so it's a useful concept to have. So basically, when we think of mineral, think of a naturally occurring crystalline structure. A mineral is not the same as a rock. We'll get

to that later, but a rock can be comprised of many different minerals, and a mineral in turn can be comprised of many different atoms and compounds. So think of those hierarchy atoms, molecules, rocks. So now that we have some idea of what minerals are, let's talk a bit about the different physical properties of minerals. This is one of the key things we want to know when we have a mineral sample or talking about a different, a new type of mineral, what are its physical properties?

So there are a number of different physical properties of minerals that are typically of interest. The crystal structure is the most basic, essentially because that defines the structure of the mineral. So all minerals have a regular crystalline lattice arrangement, and that means that there must be a certain symmetry to the arrangement of the atoms or compounds that make up the mineral. Symmetry is really a mathematical concept, and you can think of it at an arrangement as being symmetrical when you can do something to it and leave it looking the same afterwards.

So for example, if you take a square, if I rotate the square by 45 to then it looks like a diamond. It's not the same. So it doesn't have a 45 degree rotation symmetry. But if I rotate by 90 degrees, then I rotate it around sort of by one full side, it looks the same again. So it has 90 degree rotational symmetry. If I just drew a random strange blob looking shape, that would not, probably would not have 90 degree rotational symmetry. Now if I imagine a circle, that has any degree rotational symmetry, no matter what degree I rotate the circle of at central axis, it looks the same. It doesn't change. it doesn't change. So you see different shapes have different types of symmetries.

And that applies to three dimensions just as well as it does to two. So different crystal structures can be described and defined by the types of symmetries that they have. This can be studied mathematically and gets quite complicated. We're not going to go into details, I just want you to get the idea that different crystal structures can be categorized according to what types of symmetries they have. And so we have a number of crystal families, isometric, tetrae orthogonal, orthorhombic, hexagonal, monoclinic, and triclinic. I won't really describe exactly what the difference between those are.

The main point to get is that they're classified according to whether the side lengths are the same and whether all the internal angles are the same. So for example, if you had a cube, all the sides are the same, all the internal angles are the same, that would be an isometric crystal structure. Trichlinic is the opposite extreme where all the side lengths are different and all the internal angles are different. So you can think of that as a sort of an irregular prismatic structure with all different sides and angles a bit hard to explain. And then all the ones in between sort of have various different combinations of same sides and all same angles, some of the

same, some different like rectangular prisms and things like that. That's the hand wavy way of explaining the difference between these crystal families. And this is important because the crystal family that a physical mineral belongs to has will have an effect on some of the other physical properties we'll see. One property of minerals that you may be familiar with, it's one that for some reason people just seem to know about it's taught in primary schools or whatever, is hardness, most scale of hardness, which runs from 1 to 10. Talc being the lowest and diamond being the highest at 10. Now, hardness is a property that defines how much a mineral is resistant to scratching. So basically, you get two minerals and you scratch one along the surface of the other, the question is, does it make a mark?

Does it make a permanent scratch? If it does, the mineral is said to scratch the other one, and therefore it's harder than the other mineral. So diamond scratches everything, so that's why it's the hardest than 10. TELC doesn't scratch anything, none of the other main ones anyway, and so it gets a value of one, and the others in between. Mo's scale of hardness is only an ordinal scale, so it just says what scratches, what, it puts them in an order, but it doesn't give any absolute value as to how hard they are. There are other scales which do that, which put them on an absolute level, but those tend to be much more esoteric.

I want to point out also that diamond is not the absolute hardest chemical substance, it's just the hardest mineral. So remember minerals are naturally occurring and we have been able to create synthetic substances which are harder than the diamond, but they're not minerals, so because they're not naturally occurring so they don't displace a diamond on that, that's gay. Another important property of minerals is luster, so that basically refers to how reflective the surface of the mineral is. How shiny, essentially. And there are different words that are used to describe luster, depending upon the exact qualities of the shininess or the external

appearance of natural, basically. Some minerals don't have luster, so that's more a dull or an earthy luster. They kind of look like, well, earthish sort of, not necessarily earth color, but that's sort of textured, not shiny appearance that Earth has. There's some that look kind of greasy, some that are metallic, some that are pearly, resinous, silky, vitrinous, which look kind of like glass, waxy. The luster of a mineral is often determined by, as you would expect, the crystal structure and the chemical compounds that make up the minerals. So there's often a lot of interesting, even quantum mechanical effects that can go into determining both the colour and luster of a mineral, which, based on how exactly the light interact,

with the mineral structure. Again, I'm not going to go into the details of that, just to give your sense of it. I recommend looking up luster or mineral luster on Google Images, so you can see some examples of these. I'll post some up onto the Facebook, as they tend to do. Another property of minerals is one of the most obvious is colour. Now, colour is a tricky one because it's typically thought of as a non-diagnostic criteria property. So what is meant by that is that when a mineralogist is looking at a mineral sample, they'll try and identify what mineral it is, what collection of minerals it might be. So the luster could be one property that they'll use, what it looks like in terms of how

it reflects the light. Hardness could be another one, and there are some others as well, like streak and cleavage and habit that will get to, specific gravity, basically how heavy it is, but colour is typically considered to be non-diagnostic. In other words, you can't tell what mineral a sample is based on what color it is in general. The reason for that is because the colour of minerals is often determined by small impurities within the mineral structure, so small quantities of iron or aluminium or something like that. Anything really can be that significantly affect the colour.

A really good example of this is ruby and sapphire. So ruby and sapphire are actually the same mineral. They're both corundum, which is actually nine on the most hardness scale. So really they're the same. They just have slightly different impurities within the mineral structure, which lead them to be red and green, respectively. They're not different minerals. So that's an example of how colour is not diagnostic.

So it's not very useful in most cases to see what colour a mineral is. Although there are a few notable examples, like gold, for example. It has a fairly distinctive colour, but for the most part, colour is non-diagnostic. Streak is something that's related to colour, but is more useful. Striek refers to the colour of mineral in powdered form, which is often different to how it looks in the body crystalline form. And that's often more useful. So a common way of determining this is to take the rock sample or the mineral sample and literally streak it along a slab of porcelain and look at what color it is. So that grinds up a small proportion of the mineral.

So you can see it in powdered form and look at what color it is. And that's often more useful than the color of the body mineral. Another property that I mentioned earlier is crystal habit. It's a little bit of an odd name, but it refers to the basically characteristic external shape of a crystal or a mineral sample. Now it differs from luster in that luster refers to sort of the shininess of a mineral. It doesn't have to be shininess necessarily, but it's how it reflects the light. Whereas habit refers to the shape. So I guess one way of putting this is that luster would really require you to look at it,

whereas habit, at least in theory, you could tell by just feeling it, I guess. So that's the difference. Often, as a novice, an unlawful, it's a novice of these sorts of things. It's somewhat difficult to discern whether the difference between crystal habit and color and luster, because they're kind of all mixed together when you just nively look at it. But when you get more practice with this sort of thing, you can distinguish the difference between color and luster and habit and some of the other properties. Anyway, crystals have a very large, wide range of external appearances.

And again, I'd recommend doing a Google search to see this. I won't talk about all of the technical names because they're hard to pronounce, but I'll just give you a sense of it. Some crystal forms for needle-like tapered projections, where they literally look like pin cushions or porcupines. They're kind of scary looking, actually. Others have almond shape, knotty sort of a structure to them. Some of them have grape-like protuberances to them, so hematite is an example of that.

They look quite odd, actually. Some are more columnar, others are more cubic in shape, look like almost a ball cube, if you're familiar with Star Trek. Some are dendritics, so they have tree-like structure. Some are hexagonal, so quartz is a famous example, they see a sort of a large, transparent or translucent hexagonal crystal that's probably quartz quartz is very really one of the most common mineral minerals in the earth's crust there octahedral structures prismatic structures some of them around there are all sorts of really interesting structures that crystals take it should be noted that

the habit of a crystal is not just determined by the mineral composition although it is determined partly by that but it's also determined by the growing environments So minerals typically form over, sort of crystal structures form over a period of time, and in order for the orderly arrangement of atoms to, or compounds to take shape, a certain amount of time is needed. If a sample of mineral is cooled very rapidly, for example, or is formed under very high pressure, it might not have the time or the sort of space to form an its normal autoly lattice arrangement. so it may adopt an amorphous structure or it may adopt a different crystal lattice arrangement depending on the temperature and pressure and the time that it has to form. So the crystal habit and structure is not always purely a matter of the mineral composition but also the growing environment.

Another important property of minerals is called cleavage. So cleavage relates to the particular way in which a crystal structure will break or preferential preferentially breaks, or cleaves, however those, planes of weakness in the crystal lattice. So this is formed by the fact that different minerals have different bonding arrangements, and some of these bonding arrangements lead to characteristic planes of weakness, so particular places or ways in which the crystal will tend to break, and this leads to characteristic planes of cleavage, which can be used to identify the minerals. So a good example of this is,

is micas. So micas have a characteristic two-dimensional arrangement, basically like layers on top of each other. And so those will tend to cleave obviously between the layers because the bonding between the layers is much weaker than the bonding within a layer. Whereas something like diamond has a three-dimensional structure, so it doesn't have that same sort of two-dimensional. So now that we've talked about some of the main properties of minerals, so there's the external appearance in terms of shape, which is the habit. There's cleavage planes which relate in particular to the crystal structure in terms of the symmetries that are present. There's hardness which relates to the inability to avoid scratch being scratched. Luster which is basically shining as color and streak.

So now that we've gone over some of those properties, I want to talk a bit about some of the different mineral classes. So there are several thousand different types of minerals that are recognized, according to Wikipedia over 5,000. It depends exactly how they've been classified. basically each mineral has its own specific chemical composition so that is a specific arrangement of a specific ratios of atoms in regular patterns constitute a mineral and if the atoms are different or in different ratios then it's a different mineral there are some exceptions to that where you can have sort of very variable proportions but basically that's that's how it works and again that's a different

to a rock which we'll get to later that don't have a specific chemical composition Now, the different mineral classes are basically determined by the composition, as you would expect, because the chemical composition in turn determines the chemical composition in turn determines the physical properties that we mentioned earlier. The Earth's crust, where basically the minerals that we know and study are found, is comprised mostly of silicon and oxygen. Oxygen, we know that's in the atmosphere, but it's also in the crust. Silicon is the element that's used to make a lot of electronic components because it's used for semiconductor. It's the main component of sand and, well, really most rocks.

The silicon is everywhere. It's just below carbon on the periodic table in terms of it's in the same group, but one road down. So that leads us to the most important class of minerals by far and away, something like 90-95% of minerals that make up rocks and in terms of by mass composition of the crust are silicate minerals. So if you go around and pick up a random rock, it's almost certainly made mostly of silicate minerals. That leads us to talk about what distinguishes the silicate mineral structure, or class, which is the silicate ion or silica. Now this is comprised of a single silicon atom surrounded on four sides in a tetragonal structure by oxygen atoms. So it forms a tetragonal pyramid, basically.

So although all of the sides are equal because it's symmetrical, it's a little bit hard to explain. If you just do a Google search for silicate ion, you'll see what it looks like. Again, I'll post this on Facebook. It's basically four oxygen atoms spread equally around the central ion. silicon atom. So a silicon ion has a charge of plus four, because it loses its four valence electrons, it has the same number of valence electrons as carbon, and then it binds to four oxygen ions which have a charge of minus two, basically because oxygen is a very strong oxidant, so it tends to grab onto electrons, so it has a minus two charge, it

has a deficiency of two electrons in its valence shell, so when it fills up its balance shell will have a charge of minus two. So when you have four oxygen atoms surrounding essential silicon ion, the entire unit, the silicate ion, will have a charge of minus four. So you can't just have a, you can't just mush a bunch of silicate ions together because the entire structure resulting structure would have an enormous minus charge which is stable so there's a few ways that silicate ions combine together to form silicate minerals and basically the the class of silicate minerals is comprised of just different arrangements of these silicate ions so the tetragonal structures

combining together in different ways with themselves and also with other with other substances or with other elements compounds. So the main different classes of silicate minerals are defined by the, I suppose you could think of it as the dimensionality of the structure of the silicate ions with respect to each other. So you can have isolated tetrahedra and in order to bounce out the charge you'll need to have positively charged ions spread throughout the lattice as well. So typically those can be magnesium or iron or other metals like that. Metals are typically positively charged when they're in the ironic form. And so that's olivine. Olivine is comprised of single tetrahedron units with the charge balanced out by cations. It doesn't have any cleavage planes because there's no characteristic planes of weakness. Pyroxene are basically single chains of silica joints together. And the way that works is if you think of, remember that we have a central silicon atom,

surrounded by four oxygen ions. Well, if you put another silicon iron next to it, it can share some of those oxygens with the next one, which can in turn share more on the other side or with the oxygen, with the silicon next to that, and so on and so on. So you can form a chain. That reduces the overall ratio of oxygen to silicon, because you're basically doubling up on using the same oxygen for two different silicon. So you don't need as many positively charged cat ions. Those tend to have, So these pyroxines happen to have two planes of cleavage at right angles because they're the single chain and then there are the two dimensions that don't have the chain structure. Amphabols are the next class, so they're double chains.

Basically the same as pyroxenes except there are two chains rather than one. Micas, which I've already mentioned, form sheets. So instead of just sharing oxygens between silica in one dimension along a line, they share it in two dimensions. So they form planes. So those have a single plane of cleavage, basically, along the plane of the, between the planes of the sheets, where the bonding is weakest. So these still need to have cations to offset the negative charge, but they don't need as many again because the oxygen ions are being shared by more silicons, thereby reducing the total amount of negative charge. That has to be offset. Next class of feldspars, so these are three-dimensional networks of silicate ions,

where the oxygens are shared in three dimensions instead of only two in the case of the sheets, in the case of the micas. Feltsbars, though, still have some cations offsetting the negative charge, because there are still some residual charge. They're in particular potassium and aluminium are fairly common in the felds bars, as well as the micas. quarts is the final class of silicate minerals and it is comprised purely of silicon dioxide so there are no cations at least in pure quartz and it's purely a three-dimensional lattice where all of the oxygen ions are shared

between surrounding silica so it's a very simple structure quartz is also the most common single mineral in the earth crust I believe and it's a very common component of rocks as I said if you find hexagonal, largely translucent crystals. Those are probably quartz, very common. And it's because its constituent silicon and oxygen are very common as well. So basically you can think about the silicate groups as starting from independent tetrahedra, which have a 4 to 1 oxygen to silicon ratio, and then a continual decline of the relative amount of oxygen as more and more oxygen is shared between silica until you reach quartz.

which has a 2-1 oxygen to silicon ratio, and therefore doesn't need any offsetting cations to balance out the negative charge. You can see in this simple example how the mineral structure leads directly to properties like cleavage, for example, because the dimensionality of the sharing of oxygens with surrounding silica, whether it's chains or sheets or three-dimensions frameworks, determines what sort of cleavage planes,

the resulting crystal has. You can also imagine how that's going to also affect things like the luster and habit and other crystal properties. So those are the silicates. They're the most common, most abundant, and most important type of minerals. So the different classes that I mentioned, olivine, pyroxene, amphibol, mias, feldspars, and quartz. But many of those have multiple different types of minerals within them. So, for example, feldsbyers come in orthoclaclase and plagoclase forms, depending on,

mostly the cations that are offsetting the negative charge and exactly what structure and arrangement there are in. And there are many, many different types of these. So these are classes, not specific minerals that I've been talking about. So silica, having discussed the main classes of minerals, there's one other specific class of silicate, which I wanted to discuss before, moving on to the non-silicate minerals. And these are the so-called phylo-silicates. They're a class of sheet minerals,

so they're sort of like they have a micro-like structure, their sheets. The reason these are important is because clay minerals are a type of phylo-silicate. Clay minerals are a particular type of phylo-silicates, which are hydrous, so they're hydrated. They have water associated with the structure. And they're particularly important

because clay minerals are characteristic of soil So soils are basically defined as clay minerals combined with organic components. We'll talk more about this, hopefully in the future episode if I talk more about soil. But I think it's interesting that even a basic understanding of the structure of silicates and the classification can give one a sense of what is different about a clay mineral and therefore why soil is so characteristic. Clay minerals have a lot of very interesting properties which make them very useful for what growing in, and suitable for plant development, basically.

We'll talk about that later, hopefully. They're also thought to have been important in biogenesis, the beginning of life. So they're very closely associated with living organisms in a number of ways. So I just wanted to mention those. Now it's time to move on to the non-silicate minerals. So again, there are many thousands of these,

but I'm just going to talk about the broad groups, classes of minerals. The main ones that I want to discuss are carbonates, sulfides, oxides, sulfates, halides, and native elements. So you can see that really the way that minerals are grouped is by a characteristic element that's present in their structures or compound. So in the case of silicates, it's the silicate ion, the S-I-O-4 structure which is found in all silicates.

In the case of carbonates, it's the CO3 carbonate compound, which is characteristic of carbonate minerals. Carbonates are particularly important. They're typically formed, well, they're often biogenic minerals. That is, they're formed by the dissolution and subsequent crystallization out of solution of carbonate, which is a compound that's commonly found in the shells of marine organisms.

So when these die, the calcium carbonate, can often be, often is dissolved and later it may recrystallize through various means, forming carbonate minerals. One common instance of carbonate minerals that people may be familiar with is found in caves. So the famous cave structures, Stelictites and Steligmites, these are carbonates. They're formed by calcium carbonate precipitating out of water and then forming these structures basically as they drip down the calcium, as the water drips on, you know, a small protuberance in a rock, the calcium carbonate can precipitate out, forming, well, carbonate minerals, rocks, basically, and then elongating the

protuberance, leading it to extend further and further down or up sometimes. So there, calcium carbonate's the main constituent of limestone and other rocks like that. And these minerals often have, they don't look quite beautiful, so marble is a form of limestone, basically. It's formed by limestone that's been subject to metamorphic pressures, loosely speaking. And it's often used in building. The trouble with carbonates, including limestone and marble, is that they react very readily with acid, which is a diagnostic criteria actually. And that means that they are quite susceptible to erosion, acid rain and other things like that. So they're actually not great as building materials even though they look quite nice.

Sulfates contain a sulfate iron which is S.O.4 to minus. They're commonly formed as evaporates so that's what's left over when salty water evaporates basically. Oxides are a very important class of minerals. They're defined by the presence of oxygen basically, particularly the O2 anion. Typically what has a very important class of minerals. Typically what happens is the oxygen is a very, as mentioned, a very strong oxidizer, so it likes to grab on two electrons, basically. And so it will tend to form compounds with elements that are reducing agents, basically,

meaning that they tend to give up electrons. And these elements that are good reducing agents, that are good of giving up electrons, are typically metals. So metals form positively charged. catars, which as I mentioned earlier, and so they form a natural pair to the negatively charged oxide compound. So oxides are typically, basically, metal catars combined, neutralized out, the charge neutralized out by combining with oxygen in various ratios depending on the charge and other factors.

So curundum is an oxide, it's an aluminium oxide, so it's actually just comprised of aluminium combined with oxygen. Many of the ores that we use to extract metals, so like aluminium for example, or iron, are oxides, typically because these metals readily combined with oxygen. So that's actually what happens when an iron substance rusts. Pure iron is unstable in atmospheric conditions, so it tends to react with oxygen to form. So many metals are like this, I'll tend to react with

atmospheric oxygen to form oxide basically and these are the ore forms that we mine and then purify to obtain these minerals. So oxides have quite a lot of economic value because of the metals that they contain. Sulfides are a class that includes sulfide so including pyrite that I mentioned earlier, that's iron sulfide. Pyright has a brassy color which looks a little bit like gold, sort of a pale yellow and so can be known as fools gold

if you see rocks that have sort of small flecks of brassy pale yellowish-looking substances that's probably not gold it's probably pyrite gold is actually mycommon

impressive and pyrite if you compare them next to which other but anyway and finally the last class I wanted to talk about our, well second last class actually halides so these include halogens fluorine chlorine iodine

as the main an ion so negatively charged metal there So, fluorite and halite, not to be distinguished, not to be confused with the class name, halide, which is calcium chloride, sorry, sodium chloride table salt. That's actually a mineral, halite, because it includes chloride, fluoride, similar, includes fluorine. These are often formed as evaporate, so again, what's sort of left over when water evaporates? they also have important economic uses, obviously. The very last class that I want to talk about are the so-called native elements. So these are probably the easiest to recognize and understand

because they have the simplest chemical structure. Native element is just a single element by itself, not bound or bonded to anything else. There are not many native elements that are found naturally, and they have to be found naturally because otherwise they're not considered minerals. But there are a few, so a diamond is one. Diamond can be found naturally, and it has a regular crystalline lattice arrangement, and it's formed completely of carbon, just carbon arranged in a particular lattice structure.

So that's a mineral in a native element. Gold and silver are two other prominent examples. They can be found in their native form. The reason that they're not typically found, well particularly gold, is pretty much never found, bound to anything else, is because they're fairly inert, especially gold. They don't really react very readily with other things like oxygen, for example. So they're just found in native form, which makes mining for gold and silver, easier in some sense than mining for iron or aluminium, which have to be extracted,

often a great cost, or historically a great cost anyway, from the oxygen and other elements that might be found in their ores. So that exhausts the major. mineral classes that I wanted to discuss. Obviously there's a lot more detail to go into there. Briefly now, I want to just look at the three main types of, well, talk about what rocks are and then the three main types of rocks. I don't want to go into, I want to go into even less detail here than I did with minerals because there's a lot more to say about rocks, in particular because different types of rocks are associated with different geological processes in a way that's not as directly relevant to minerals.

And so they're more readily discussed in detail when I discuss, for example, volcanoes in the context of igneous rocks and various erosion and deposition processes when I discuss sedimentary rocks. Tectonic plates when I discuss metamorphic rocks. So I don't want to go into a very large amount of detail here because they're naturally discussed in other settings. But I do want to introduce the concept of rock and briefly discuss the main classes. So what is a rock? I mean, we think we all know what rocks are. Interesting thought exercises to imagine, hey, we define a rock to someone who's never seen one before.

Not as easy as you might think. So in geology, a rock is a naturally occurring solid aggregate of one or more minerals or sometimes mineraloids, which are basically mineraloids are substances that meet some, but not all of the four characteristics of minerals that we mentioned earlier, but I'm not too interested in that. Basically, they're formed it. So basically rocks are solid aggregates formed of minerals.

Aggregant means they're sort of just combined together. Rocks don't have a specific chemical composition. So if I tell you this is a mineral, you can legitimately ask me to write down the chemical formula for that mineral. Because it must have one if it's a mineral. Sometimes it's simple, sometimes they're very complicated, but they must have one. If I ask you to write down the chemical formula of a rock, you would legitimately just look at me confused because rocks don't have, or at least in general, don't have specific chemical formula.

That's the main difference between a rock and a mineral. Rocks can be made out of many different minerals. So there are sort of a level up in organisation, as they mentioned earlier, from minerals, if you want to think of it that way. Of course, a rock could be made of a single mineral. A rock could even be made of a single element. You could have a rock which is basically pure 100% gold, for example, a gold nugget. That's basically a rock that's only one element.

Those are pretty rare, though. But it's possible. Now, as I mentioned, there are three different types of rocks, igneous, sedimentary and metamorphic. Now, again, this may be slightly heretical. It's my opinion that the boundaries between these three types of rocks are more conventional than absolute. So, as we'll see, hopefully in future episodes, if I get around to talking about the processes that go to forming sedimentary, metamorphic, and igneous rocks, really there's a bit of a fuzzy boundary between them.

There's a bit of a fuzzy boundary between a sedimentary and a metamorphic rock and metamorphic and an igneous rock. But nonetheless, it's still very useful to talk about the three classes, because in most cases it's fairly clear which class a given rock falls in. And the igneous rock, comes from Latin word for fire, forms through the cooling and solidification of magma or lava, or lava, or lava, or lava, or lava, or lava, or lava, or lava, or lava, or lava, or lava, or lava, or is just molten rock. The particular structure and composition of an igneous rock is determined mostly by the temperature and pressure at which it solidifies and also the time span at which it solidifies or crystallizes. Most igneous rocks are comprised mostly of silicate minerals. That applies to most rocks I suppose, but especially igneous rocks because most of the crust prized of silicates, so it's not surprising that the rocks we get out of it are when the magma crystallizes are highly silicate enriched. Mostly the classification within igneous rocks is dependent about how much silicate is in it. So so-called granitic rocks are relatively

enriched in silica, so-called basalt rocks are relatively deficient in silica and antacitic rocks are in between. When I say enriched and deficient, it's important to bear in mind that even basalt basaltic rocks have like 50% plus silica, usually. It's just that antacitic and granitic rocks have even more like 60, 70%. So it's to do with how much silica, but they all have a fairly large amount of silica. That's why I said if you just pick up a random rock,

it's probably mostly made of silica, of silicate minerals because they're just that common. So volcanoes, when they spewate lava, and that crystallize and solidifies, that's an igneous rock. Moving on to sedimentary rocks. These are the most common rock that you probably see if you just pick up a random rock on the earth's surface, although it does depend where you live.

Sedimentary rocks are formed by sedimentary processes. So they're formed on the earth surface by the accumulation and sedimentation of fragments of other rocks and minerals and sometimes organisms as well. So fossils are only found in sedimentary rocks, for example. Centimatory rocks are always made of previous rocks, So ultimately you can think of everything derived from an igneous rock at some point, I guess, if only because if you go far enough back in time, all of the surface was molten and there were no solid rocks. So the first of those that crystallized were the first igneous rocks, and then when weather processes began to break up those igneous rocks and transport them and reform, sediment them and combine them together into the first sedimentary rocks, then we have the first sedimentary rocks and so on.

The classification within sedimentary rocks is mostly determined by the size of the particles that come together to form sediment, so anything from big boulders to tiny clay particles and in between, and also the process that I guess combines them together. So you can have chemical precipitants, so basically the water evaporates, leaving chemical evaporate rocks behind, or you can have temperature and pressure, you can have plastic sedimentary rocks, so there are different processes that can lead to sedimentary rock formation. But sedimentary rocks are basically products of erosion and transportation through weathering processes, in particular wind, water, ice, glaciers and also mass movement, so mass wastage, that's the gravity basically, and also biological processes. So tree roots, for example, can break up rocks and animals can dig and things like that. So all of these processes lead to the formation of sedimentary rocks. Fossilization, as I mentioned, also considered sedimentary rocks. Final type of rock, the metamorphic rocks. Menomorphic rocks are formed by subjecting any type of rock, including sedimentary igneous or another metamorphic rock to sufficient temperature and pressure,

which lead to a change in the, not the chemical composition, but the chemical structure of the rock. So, metamorphism basically means change in form, and that's what happens to a metamorphic rock. It changes in form, but generally not in composition. Now, it's important to note, if you subject a rock to high enough temperatures and pressure, it will melt.

If it melts and then re-crystallize it, it's an igneous rock. It's not a matter. a metamorphic rock. In order for a rock to be igneous, it must melt. If it does not quite melt, if it's subjected to temperature and pressure, which can cause it to become, start to flow, to become plastic, but doesn't melt, then it's, then it gives rise to what we call a metamorphic rock. So you can see how there's scope for sort of some, a grey zone where it, like, partially melts, but doesn't completely melt. So the classification of metamorphic rocks is usually based on how metamorphized, they were. We talk about the grade of metamorphism. So basically, the higher

temperature and the longer that they were subjected to it, the more highly, the higher the grade of the menomorphic rock, and the more foliated they tend to become. Foliated meaning that you can observe distinct bands within the rock, and that tends to, those tend to occur as a result of significant pressure and temperature. There are non-foliated mammothic rocks like marble, but many metamorphic rocks are foliated. So if you see those foliations, that can be assigned in metamorphic rocks. And the more distinctive they are, typically the higher grade they are. So those are the three classes of rocks, and a little bit about how they form and how they're classified. There's obviously a lot more to say there, which hopefully will be left to future

episodes. So before wrapping up, there's just one brief little final topic that I wanted to discuss, which relates to how the different rocks are, rock types are related to each other. This is called the rock cycle, which you may have heard of before. It's also something that seems to be taught quite a lot. So the rock cycle just explains the relationship between the different types of rocks. Now, it's important not to think of it as some sort of determinant cycle. Rocks don't have to go around in any one direction or necessarily will ever move beyond one form that they're in. I mean, it depends on what physical processes are operating, what environment they're in.

The point is, though, that rocks or minerals that comprise them, can cycle between a number of different forms, basically magma, igneous rocks, sedimentary rocks, and metamorphic rocks, and there are particular processes that interrelate those forms. So, we've already mentioned a number of them. We'll start with an igneous rock. Actually, we'll start with magma. That's a more natural standpoint, a starting point, because that's the form that minerals take in the Earth's mantle, or at least parts of the earth's mantle.

So magma, basically liquid minerals. When those cool, they crystallize, solidified to form an eagnus rock. Now, if you melt that, it can go back to form a magna, a magma, or if you heat it and subject to a sufficient pressure such that it doesn't melt, but it changes its structure or a form, that forms a metamorphic rock. And you can, of course, melt a metamorphic rock back to a magma. You can melt anything back to a magma.

Although typically you don't melt a sedimentary rock directly to a magnet because first it will metamorphies and then it will melt. We've got, at the moment, igneous and metamorphic rocks, which can melt to magmas. Magma can't cool directly to a metamorphic rock because it cools to an ignes rock, which then, subject to heat and pressure, can become a metamorphic rock. Well, what about sedimentary rocks? How do they fit in? Well, both metamorphic and igneous rocks, either of those can be subject to weathering and erosion effects, which form sediments.

Sediments themselves aren't rocks. They're sort of fragments of rocks, if you want to think of it that way, which then, through further processes, sedimenting and compactation processes form sedimentary rocks. Sedimentary rocks in turn can also be subject to weathering and erosion to form new sediments and then new sedimentary rocks. Or they can be subject to heat and pressure to form metamorphic and then finally melt to form magma. So that's the relationship between the three different rock types. And again, this shouldn't be thought of as a deterministic cycle. I mean, a given mineral substance can just sit as a metamorphic rock for a billion years. And, you know,

that's perfectly fine. But it just describes. the relationships between the different types of rocks. And there's obviously a lot more to say there about how and when each of these processes takes place, but that's for a future episode. So that concludes what I wanted to say today. I wanted to finish out the episode with just a brief announcement, I suppose, or explanation about the podcast in its future. So you may have noticed that there hasn't been an episode in about six months.

That's pretty poor form on my part. I apologize for that. The reason for this is that I've been unusually busy lately, and unfortunately, the podcast has had to take a back seat. When I first started the podcast, which is over five years ago now, unbelievably, I wasn't as busy as I am now. I didn't have as many additional commitments. So at the moment, I'm still studying, and I'm actually taking on more subjects than I had in the past over the summer and other times like that, which keeps me busier. But I'm also working part-time. I'm involved in a couple of clubs at university and elsewhere. I'm writing,

I write it on a blog periodically, and I'm also, my newest project is I'm writing a book. This book unfortunately doesn't really have much to do with the podcast, although there might be one or two episodes I could get out of it, because some of it does relate to cosmology. So for the most part, the reading that I do for the blog and podcast are not related to each other, or the book in the podcast. The book and the blog that I write are mostly related to, basically, philosophical questions, and particularly about religion which I'm quite interested in. You can check out my blog at thegodlesstheist.com

if you're interested in that. But unfortunately it doesn't relate directly to the podcast, so it means that there's a certain diversion of resources. Now, what I will do is ensure that the past episodes are always accessible on the website, so those are always going to be up. So I encourage people to go back there, listen to past episodes, there's a wealth of material there. The episodes are designed to be timeless.

There's no real news material or anything that gets out of date, there for the most part. And also there is plenty of content within each episode. So I've designed them so that they can have relistening value, particularly in her six months a year later. You can relisten to them. It's a good way of revising material. The podcast are really designed to be a learning aid and to help people be exposed to new knowledge. So that's what I've made them for. So please continue to take advantage of them and stay subscribed if you are. There's no cost to staying subscribed. In terms of the future of the podcast, in terms of new content, I've decided that what I'll do is commit to producing a new episode every three to four months,

which is not especially regular, I know, so it can be thought of as a continuation of the current quasi-dormant status. But it's not completely dormant. I want to have a little bit of new content dribbling out occasionally. Please feel free to continue emailing me. Someone mentioned, brought to my attention, that one past episode wasn't working properly in terms of downloading it. I still don't know why that happened, but I was able to re-upload it and fix that. So if there are any issues like that, please bring them to my attention.

You can contact me at Fods12 at gmail.com. That's FODS12 at gmail.com. Thank you for your continued support. I do hope in the future maybe year, two years down the road to continue more regular production of episodes. If I have more time and some of my other projects, free up some time for me. I have many more episodes that I'd like to produce. So there's easily another 100 episodes that I could do in terms of content. But it does take a certain amount of time, which I just don't have at the moment, unfortunately.

So, thanks for listening. Talk to you next time.