Euhedral Crystals? EarthCache
Hidden : 10/8/2014
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:

Prominent buildings like the Midland Hotel, Manchester as well as being iconic parts of modern day life often display interesting geological features. Here is one of those features so easily bypassed


A Fred Broadhurst Memorial Earthcache

Please note that this is an Earthcache; a find will only be accepted if the Logging Requirements have been met.
Midland Hotel The ground floor facade of the Midland Hotel's external superstructure is mainly polished Granite and affords non-geologists with an insight into Igneous rock.

The Granites For decorative purposes two different Granites are employed, a light coloured pinkish granite from Peterhead is divided by a dark layer of Shap Granite; it is the latter layer of Porphyritic Granite which is the subject of this Earthcache. The adjective 'Porphyritic' simply applies to the texture of the granite; large crystals of a mineral are surrounded by a matrix of smaller crystals and the relative sizes of the two were defined by the rate of cooling of volcanic magma (molten rock) forming Igneous Rock

Phenocrysts Clearly visible in this border of Shap Granite are Phenocrysts. discernably large crystals in a matrix of smaller mineral crystals. These large pink crystals are Orthoclase Feldspar and would have been the first crystals to solidify as the magma cooled on reaching the earth's surface. It seems that the cooling process at the time was not a constant process, but starting slowly allowing the Phenocrysts to grow then later more rapidly to allow the other minerals to solidify. These other minerals are typically in Granites, glassy Quartz, white Plagioclase Feldspar. and black Biotite Mica. and can be seen at gz. For the purpose of classification of rocks and minerals crystals can described as having, clearly defined crystals Euhedral, no obvious crystalline structure Anhedral, or somewhere in between Subhedral.

Carlsbad Twinning One of the features of slow cooling which produces the larger Phenocrysts is Twinning. Small foreign particles at the interface between liquid lava and the newly forming crystal not only act as nucleii to the growth process but can in some cases cause the growth to take place back to back. This can be seen often as a line dividing the crystal into two halves parallel to the sides of the crystal; this type of twinning is known as Carlsbad Twinning. (Another type of Twinning, 'Baveno twinning' where the twin plane is diagonally across the crystal is much rarer but might be present in this Shap Granite)

Logging Requirements
1. Examine the dark layer of Shap Granite and decide whether the Phenocrysts are Euhedral, Subhedral or Anhedral.
2. Do the same for the rest of the crystals.
3. On the Moh scale of hardness Feldspars are 6 and Quartz harder at 7, but the other common mineral in the Shap Granite, Biotite Mica with a hardness of 2.5-3 is significantly softer. What effect does this variance in hardness have on the suitability of the Shap Granite as a building material?
4. On a rectangular Orthoclase Phenocryst, is the Carlsbad Twin plane parallel to the long axis of the crystal or the short?
5. Approximately how much bigger are the Phenocrysts than the other minerals?
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Additional Hints (Decrypt)

Svaq qryrgvba? Cyrnfr ernq yvfgvat

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)


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