Introduction
Many of my EarthCaches are in towns and cities where exotic rocks are abundant, and impressive, historic architecture is commonplace. Sadly my home town is somewhat lacking in these things - or so I thought...
Sometimes it's surprising what there is to see, that we pass by without noticing. There's not a great deal of impressive architecture but there are various examples of granite to be seen, dotted here and there.
Some of these granites are used as cladding on the facades of buildings, to a greater or lesser degree. Some of them are used in paving features and even as rain gullies and kerb stones. In fact the amount of granite used in kerb stones in Chorley probably outweighs the granite used for more obvious decorative features - so it's perhaps slightly ironic that kerb stones don't feature in this particular EarthCache (although they may do in a future one 😀).
Part of the raison d'être for this particular EarthCache is to show you different types of granite with differing colours and crystal sizes as well as polished and unpolished examples. The greater theme of the EarthCache though is the identification of a particular type of crystal, known as a phenocryst, which is sometimes found in granite, as well as another feature common to igneous rocks known as an enclave.
You'll need to visit several locations to see all these examples - but they are all within a fairly small radius so your shoe leather shouldn't incur too much wear-and-tear 😉.
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There are six sets of coordinates - the main coordinates and five additional waypoints. Visit each one in turn and tell me - for each location:
- What combination of colours are in the granite here?
- Are there any phenocrysts visible in the granite here?
- If phenocrysts are present here, what shape and colour are they?
Also:
- One of the granites has enclaves - which one is it?
- Please provide a photograph of yourself - or a personal item - at one of the locations as proof of your visit.
- Feel free to include with your logs any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
Background
Granite is an igneous rock which makes up the bulk of Earth's continental crust. In fact, research indicates that Earth is quite special in this regard as it's the only planet in our solar system where granite is found!
There are lots of igneous rocks which get called granite because they look granitic, but granite has a rather specific recipe.
Granite is composed mostly of two minerals: quartz and orthoclase feldspar (a potassium-rich variety of feldspar). Quartz must make up at least 20% of the rock and orthoclase at least 35%.
If either of those criteria is not met, then the rock is not granite. In fact, those are the only two minerals that have to be in the rock! The remaining rock (up to 45%) can be one or more other minerals, such as plagioclase feldspar (a sodium-rich variety), hornblende, pyroxene, muscovite, or biotite (the last two are kinds of mica).
The bulk of the minerals in granite are light coloured - pinks, browns, white or clear - in the case of quartz - which is why most granites are light-coloured rocks. The darker minerals usually make up only a small fraction of the rock, adding a speckled aspect to the overall appearance of tightly interlocking crystals.
Born of fire!
Igneous rocks, such as granite, are also known as fire rocks because their formation involves temperatures as high as 600 to 1,300 degrees Celsius.
At these temperatures, solid rocks melt and become, in simple terms, a sort of mush of various minerals which is known more scientifically as magma.
First up - best dressed
If magma remains deep underground it cools very, very slowly and as it does so, particular minerals within the magma start to crystallise, becoming solid once again. Some minerals are able to crystallise at higher temperatures than others, so those minerals effectively get a head-start on forming nice, regularly shaped, sharply defined crystals with features such as straight edges 😍. This head-start gives them longer to grow in size and also to grow without struggling for space and being squashed - because the other minerals in the magma are still in their soft, melted state.
Each mineral has a particular chemical formula which dictates, at the atomic/ionic/molecular level, the shape of the crystal lattice that will be formed as that mineral crystallises. In turn, the shape of a particular mineral's crystal will reflect the shape of its crystal lattice - repeated many times over as the crystal continues to grow in size. This phenomena is also known as crystal habit.
Those crystals which form earliest, when there's the greatest amount of room available for unimpeded growth, tend to be the ones with shapes which most closely fit the crystal habit belonging to the mineral they are made of.
Later on, as the magma continues to cool and falls to temperatures at which other minerals can start to crystallise, things get crowded, space becomes limited and crystals have to fit into what space is available - so they end up squashed, irregular in shape and interlock with each other very tightly - which is one of the reasons why granite is such a strong rock.
Crystal shapes
Given adequate space to grow unhindered, crystals will tend to form in particular regular shapes specific to the chemical makeup of the substance they are made of and such crystals will tend to have regular, well defined faces, edges and angles, dictated by their crystal lattice. Below are examples of a few typical ideal crystal shapes:
Remember though that the crystal shapes shown above are three dimensional, but when you look at each example of granite you'll see in the course of completing this EarthCache, you're looking at a flat face and will see only two dimensional shapes - and that those shapes will depend on the angle of the cut through the granite to form slabs and blocks.
Phenocrysts vs Groundmass
Phenocrysts are crystals which are conspicuously larger than surrounding crystals in an igneous rock. Igneous rocks which feature phenocrysts are described as porphyritic.
The collection of crystals making up a porphyritic rock that are not phenocrysts is known as the groundmass. If we think of the phenocrysts as being the stars of the show, so to speak, the groundmass can perhaps be thought of as the background.
I don't know if there's an official ratio of crystal sizes that differentiates between phenocrysts and groundmass but I did read one paper which suggested a ratio of 5:1 i.e. phenocrysts being at least five times larger than the surrounding groundmass crystals - and I think that's a reasonable guide.
To help you to recognise phenocrysts when you see them, here's a little collage of a few photographs I've taken in the past of phenocrysts of differing shapes in various granites - squares, rectangles, round ones, rhomboid ones - even crystal twins (bottom image of middle row):
In those shots where you can see my finger (for scale) it's touching the groundmass crystals, pointing out the much larger phenocryst, except perhaps in the largest image in the collage which shows a type of granite in which the phenocrysts dominate.
Porphyry = Porphyritic
As an aside, and for completeness, the phenocrysts that you'll see in the course of completing this EarthCache are plenty large enough to be easily recognisable - but they needn't be of this scale to be classed as phenocrysts. Remember - they need to be conspicuously larger than the surrounding groundmass.
Below is a section of a photograph that I took on a rainy day in Blackburn of some Porphyry paving blocks - with oak leaves either side for scale 😄.
This Porphyry has a very fine groundmass so the phenocrysts - the scattering of obvious light-coloured crystals in the photograph - are tiny compared to the ones you'll be looking for. But they are still phenocrysts because they are conspicuously larger than that very fine groundmass 😉.
NOT Phenocrysts
There are other distinctive features which appear in granite which might be mistaken for phenocrysts, so I thought I'd show just a couple of examples here to aid with identity.
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Crystal clusters - I took this photograph of some granite the other day, with a dark mass in the middle which is larger than the obvious pink feldspar and glassy quartz groundmass crystals surrounding it.
That dark patch though isn't a phenocryst. It's actually just a collection of crystals of mica - crystals which are in fact smaller than the majority of surrounding groundmass crystals.
Another clue which makes it easy to recognise that this isn't a phenocryst is that the edges of the cluster are all uneven, indicating that what we see here is a mass which grew when available space was limited.
Remember that when phenocrysts are developing there's typically more space to grow in the regular, well defined shapes associated with the mineral's characteristic crystal matrix.
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Enclaves - another photograph that I took recently of a large, dark, ovoid mass embedded in a light-coloured granite.
That large, dark, ovoid mass is known as an enclave and it's made up of lots individual small, dark crystals. The ovoid shape is typical.
In simple terms, enclaves develop when magmas of two different types become partially mixed. Here a blob of mafic magma (rich in magnesium and iron) has found its way into a mass of felsic magma (rich in elements that form feldspar and quartz) before the whole mass cooled and solidified.
Mafic magma is more viscous than felsic magma and this may account for the limited mixing of the two.
If you've carefully read and digested the information from this cache page - INCLUDING THE DETAIL IN THE ADDITIONAL WAYPOINTS - your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there 
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Please submit your logging task responses before posting your log.