Introduction
The thing about geology is that there seems to be a never ending stream of new words to learn.
The terminology which geologists use to describe things can seem incredibly complex to the the point of being almost impenetrable for those of us who aren't experts - but learning some of it is a fairly essential part of expanding our ability to describe and categorise what we see in the field.
This EarthCache deals with a group of three common terms used to describe crystal shapes. The particular crystals in question are found in a monument made from a nice example of Dartmoor granite which formed around 280 million years ago.
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In the cross there is one very obvious Euhedral crystal which is by far the largest of all the crystals in the structure. Based on your study of the cross and using the information on the cache page please tell me:
- What colour is that very large euhedral crystal?
- What does the crystal measure from corner to corner? (Make sure you measure between the two directly opposite corners which are furthest from each other).
- What shape is the crystal i.e. triclinic, hexagonal, trigonal, or something else?
- Optional task: feel free to add 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
At the given coordinates you will find a tall market cross carved out of Dartmoor granite.
Granite is an igneous rock formed when magma (molten rock) cooled slowly and crystallised deep underground. The resulting rock is very hard and is made up of tightly interlocking crystals of a number of minerals which are visible with the naked eye.
In some granites all the crystals are roughly the same size. Geologists describe these granites as having a phaneritic texture. Other granites include some crystals which are much larger than the rest. Geologists describe these granites as having a porphyritic texture. The Dartmoor granite has a porphyritic texture; it has some large crystals of a mineral called orthoclase feldspar. These larger crystals are surrounded by a mixture of smaller crystals of other minerals, including plagioclase, quartz and mica. Geologists call this surrounding mixture of smaller crystals the groundmass.
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. Below are examples of a few typical ideal crystal shapes:
The 'hedrals
Some crystals form in ideal conditions and achieve their characteristic shape. Some crystals form in less than ideal conditions and only achieve partial perfection. And then there are crystals which form in cramped conditions and they can be all sorts of shapes 
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Geologists have special words to describe each of these crystal forms:
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Euhedral - crystals are crystals which have been able to grow unhindered into their natural, regular crystal shape - as opposed to having their shape altered by being forced to compete for space with the crystals surrounding them in the molten rock.
VERY IMPORTANT to remember though that euhedral does not dictate the crystal shape - only that the crystal fully achieves the ideal shape for the type of crystal it is - with smooth, well defined and easily recognisable faces, edges and corners.
A euhedral crystal of the mineral halite for example (more commonly known as rock salt) would be a well defined cube - with smooth, well defined and easily recognisable faces, edges and corners. A crystal of quartz on the other hand would be hexagonal in nature and so long as that crystal had well defined and easily recognisable faces, edges and corners it too would be described as euhedral.
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Subhedral - crystals are 'less than perfect' in that some of their faces / edges / corners are well defined - but not all.
It might be that a crystal started off euhedral, because it initially had plenty of room to grow, but later got squashed by other crystals pressed up against it. Equally it may have been that from the outset of crystallisation there was enough room for some of the faces of the crystal to grow in a well defined manner but other faces were deformed by adjacent crystals which were taking up available space.
In the image on the right the left-hand-side of the crystal is less well formed than the the right-hand-side. The right-hand-side is closer to euhedral than the left, but overall the crystal as a whole is subhedral.
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Anhedral - crystals have a pretty tough life . The space they have to 'grow up in' is pretty limited and there's lots of competition from surrounding crystals - so they have to fit into what space they can.
Perhaps ironically though it might be argued that the flexibility of anhedral crystals actually contributes significantly to the overall cohesive strength of the granite, as tightly interlocking crystals are a fundamental part of granite's strength.
Different minerals crystallise at different temperatures and, as a rule of thumb, anhedral crystals tend to be of those minerals which form at the lower temperatures which occur later in the process of a magma cooling and becoming solid rock.
If you've carefully read and digested the information from this cache page 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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