Quartz Vein(s) - Amazing Geology on Guernsey

Quartz is often found in veins that cut through rocks. Although the term "vein" suggests this, the veins of quartz and other minerals are usually not thin tubes, but rather thin sheets. The veins can form under various conditions, and depending on these conditions, may or may not contain quartz crystals.

Even though certain types of quartz veins do never bear any crystals, it sometimes makes sense to follow large quartz veins to look for crystal-bearing fissures: Should a rock that contains old large quartz veins have been folded later due to tectonic forces, the quartz veins represented a disturbance (a discontinuity in the otherwise homogeneous mechanical properties), and alpine-type fissures are likely to open up between the quartz and the host rock.

The dimensions of the veins vary in width and length, from centimeters to decimeters thick and from decimeters up to several meters long. Quartz veins are mostly fracture related and have clear contacts with their host rocks.

The simplest type of a quartz vein is the filling of an already present crack in rocks. The crack might form during folding of the rock in mountain-building processes, by shattering during tectonic events, by a decrease in pressure during the uplift of a rock, or because a rock cools down and shrinks. Hot brines that percolate the rocks and originate at greater depths with higher temperatures will precipitate the minerals they carry with them in cracks at lower temperatures and pressures. This process may continue until the crack is completely filled or may stop before, leaving "pockets" in the vein that are sometimes outlined by crystals. Hot brines that enter a crack in the rock from some distant hot source like a granite pluton first cool and precipitate most of their load rather quickly. The result is milky quartz, either massive or made of interlocked milky quartz crystals. Later, when the crystal growth slows down, the crystals may get less milky or even clear. In the majority quartz veins, most of the quartz is precipitated as massive, milky quartz. Well-formed crystals, if found at all, are only a small portion of the vein filling.

These veins almost always run parallel to the major fracture zones of the large granite stones. Example: Vertical quartz veins in a coarse-grained red granite at Cala Sarraina beach,

A close-up view of one of the veins reveals that these are made of intergrown milky quartz crystals. A free standing quartz crystal can be seen sitting in a small pocket in the lower part of the image.

Rocks are sometimes cut by quartz veins at random orientations, with or without crystals. In other cases there is an obvious pattern in the orientations of the veins: for example, when a igneous rock shrinks during cooling, the cracks may develop into a regular system of fissures that might be filled with quartz from percolating hot brines. The image shows a small pocket with small quartz crystals in an irregular vein in a granitic rock at the Kingston Mountain range in southern California

Because hot brines that enter a crack cool rather quickly and precipitate a lot of material in that early phase, many veins show a symmetric zoning. The outer parts bordering the host rock are mostly made of milky quartz while the core is made of more translucent quartz or even clear crystals.

 

Other Examples of Quartz veins

Sources
wikipedia.de
www.sciencedirect.com/topics/earth-and-planetary-sciences/quartz-vein
quartzpage.de
www.alexstrekeisen.it/english/meta/veins.php

So lets take a closer look of the geologic formation at the coordinates.
To log this Eartcache, go to the given coordinates and answer the following questions. This via email or the message center to me:

1. a) How many veins do you find in the area at the given coordinates?
   b) Describe the texture, the color(s), and the average width of the veins?
   c) How is the course of the vein(s)? Are there clear straight or more curvy?
   d) Can you see branchings in this vains or are there are only singular?

2. Do you see any single free standing/laying quartzs chrystals?

3. In the listing is decribed, that veins often are not tubes rather than sheets. Do you any evidence for that statement. If yes, describe your discovery

4. Take a closer look at the edge of the quartz vein to the surrounding rock. Here you can see differences in structure and color. What is your assumption as to what could be the reason for this (possible keywords and ideas for your considerations: erosion, age of the vein, direction of formation and growth, vein in vein due to new fissure fracture)?

5. What kind of rockdo you think is the surrounding rock, which keeps the veins?
   
   Additionally

6. Take a photo of yourself and/or a personal item (e.g. your GPS) and attach it to your log!