Silicon dioxide - DP/EC 12one EarthCache

Geological background

The Devonian (Frasnian) Portscatho Formation is dominantly composed of alternating grey or greenish grey, sandstones and mudstones (slates) along with less abundant siltstones. The Mylor Slate Formation is well developed around Carrick Roads and is dominated by dark grey slates with thin pale grey sandstones and siltstones. This rock unit is also Devonian (Famennian) in age. The Porthleven Breccia Member is included within the Mylor Slate Formation, but comprises angular fragments of a range of rock types included within mudstones. Locally igneous rocks (originally basaltic in composition, but altered during regional metamorphism to what are referred to in Cornwall as greenstones) occur in amongst the Devonian sediments. All of these Devonian rocks were folded and regionally metamorphosed during a major mountain building event known as the Variscan orogeny some 350-290 million years ago. 

Subsequently large granite masses were emplaced into the Devonian rocks. Two large granite bodies occur within the catchment area of the Fal Estuary; the Carnmenellis Granite to the west and the St. Austell Granite to the east. The Carnmenellis Granite is roughly circular in outline and is predominantly composed of muscovite-biotite granite. To the east of the estuary is the St. Austell Granite. This granite is composed of three main textural and lithological varieties; the eastern part is largely composed of biotite granite, whilst the western part is composed of topaz granite and tourmaline granite. The tourmaline (or lithium-mica) granite is extensively altered to kaolinite. The topaz granites occur as well defined bodies within the St. Austell Granite. (SOURCE: HERE)

The Portscatho Formation is also dominated by several quartz veins. These veins are typically exudation quartz that is emplaced parallel to the existing foliation. In the rock, it contrasts by the very white colours against the more greenish and brown sediments and is therefore very easy to identify.

Quartz

Quartz is a mineral composed of silicon and oxygen atoms in a continuous framework of SiO₄ silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO₂. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.

The word "quartz" is derived from the German word "Quarz", which had the same form in the first half of the 14th century in Middle High German in East Central German and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý ("hard").

To validate your found in this EC, you have to get to the GZ coordinates and answer the few easy questions:

1. What is the average width of the quartz veins? (< 0.1m; > 0.1m?)
2. What is the average length of the quartz veins (< 1m; > 1m?)
3. Are the veins regular or irregular?
4. Are they fairly straight or severely folded?

(PLEASE: No photos in logs that will give away the answers!) - logs with spoilers will be deleted.

The most exciting way to learn about the Earth and its processes is to get into the outdoors and experience it first-hand. Visiting an Earthcache is a great outdoor activity the whole family can enjoy. An Earthcache is a special place that people can visit to learn about a unique geoscience feature or aspect of our Earth. Earthcaches include a set of educational notes and the details about where to find the location (latitude and longitude). Visitors to Earthcaches can see how our planet has been shaped by geological processes, how we manage the resources and how scientists gather evidence to learn about the Earth. To find out more click HERE.