Migmatite - Amazing Geology on Jersey

Migmatite is a partially melted metamorphic rock. The name migmatite is of Greek origin and translates as mixed stone - a reference to the composition of migmatites. Migmatites show characteristics of both gneisses and/or slates - metamorphic rocks - and igneous rocks such as granite. Migmatites - when rocks begin to melt: When an average rock is heated, it doesn't behave like a piece of ice, which simply melts and loses its shape in the process. Rocks melt in stages because they are made up of different minerals, each with their own different melting points. When heated, the light-colored minerals (quartz or feldspar) liquefy first because they have the lowest melting points. The surrounding framework of minerals remains still tight. As the temperature continues to rise, the mineral with the next higher melting point then liquefies and so on, until finally all the minerals are liquid. In this process, a wide temperature range is passed through. The melting point of a mineral depends on several factors. The presence of water and CO2 has a major impact. Both dramatically lower the melting points of the other minerals just by their presence. Likewise, different minerals influence each other, i.e. the mere presence of one changes the melting point of another. In addition, the pressure within the rock plays a crucial role. At high ambient pressure, minerals melt later, i.e. at higher temperatures. At low pressure, melting starts at lower temperatures.

Even if the whole process is very complex in detail, the basic processes remain the same and can be observed again and again. One such constant is that in rocks, the light-colored minerals quartz and feldspar melt first. A migmatite is a rock in which this partial melting had started, but did not finish, but got stuck. A migmatite stands between a gneiss (deformation, but no melt) and an igneous rock that was completely melted and then cooled. Many migmatites show vivid markings and can be downright colorful. Occasionally the rock appears kneaded or stirred. In addition, migmatites are extremely tough and solid rocks. That is why they are valued stones. Macroscopically, migmatites are very diverse in appearance, some with a metamorphic structure and some with a magmatic appearance.

According to Mehnert (1968), migmatites are divided into four structural elements:

The paleosome is that part of a migmatite that did not undergo partial melting and therefore contains the genetically older solid phases of the rock. Nevertheless, the paleosome can react chemically with the molten rock during melting. Unlike the mobile melt, the paleosome is stationary. Its composition and texture reflect the highly metamorphic parent rocks that existed before the Anatexis.

The neosome is the formerly liquid, melted phase of the rock, i.e. the younger formation. Is the neosome rich in quartz and feldspars, e.g. B. plagioclase and alkali feldspar and thus of lighter (usually red) color, it is called leucosome. The leucosome shows a magmatic structure.

If it is dark in color and therefore rich in cordierite, biotite and hornblende, it is called a melanosome. These parts usually form a dark border around the leucosome.
Since the melanosome is the hard-to-melt remnant of the parent rock from which the leucosome emerged, it is also referred to as a restite. Due to the alternating arrangement (vein, layer, nest-like, also streak-like or hazy) of leucosome and melanosome in the cm to dm range, the migmatites often have very different and striking structures.

typical textures: a) agmatitic, b) dictyonitic, c) stromatitic, d) folded layers, e) schlieren texture, f) ptygmatic folding

Sources
wikipedia.de
mineralienatlas.de
www.kristallin.de/Metamorphite/Migmatite-druck.htm
www.steine-und-minerale.de
spektrum.de

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. At the coordinates you will find a very colorful rocky landscape.
   a) What colors of the rocks can you see at all? What colors have the migmatites
   b) How does it feel (soft or rough)? Can you see individual structures or additional chrystals in the rock.
   c) If you look at the picture in the listing with the different textures. Which one(s) can you find here?

2. If you think about the description in the listing, you will find various migmatites here. Which classification according to Mehnert can you see here?

3. Are the bands uniformly flat or can you see structures? If you think about the formation of migmatites by melting, what could be the reason? (Possible keywords for brainstorming: melting point, degree of hardness, erosion)
   
   Additionally

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