🌎🌎🌎🌎🌎🌎Gneiss (Nice) Is It?!?!🌎🌎🌎🌎🌎🌎 EarthCache

A cool find while in the area.

We decided to create a short easy study of these Nice Gneiss.

(Yes that is how to pronounce it... isn't that nice?...lol)

Gneiss- definition

This rock is a metamorphic rock from either igneous or sedimentary origins. These parent rocks are put under extreme heat and pressure that transforms into this Gneiss. Depending on the minerals that are present the outcome of the the gneiss can be very nice like you see here. Rather splashy looking. In this instance this rock is from the area near Burnet where Precambrian Rocks have been sourced. This means that these are some of earths oldest rocks. Precambrian was fromed 3 billion to 600 million years ago before the Cambrian Period.

Common characteristics of Texas Gneiss are;

- it is banded or layered. (a result of the high pressure and heat that metamorphosed it)

- it is foliated. This means that under the high heat and pressure the minerals present were stretched and spread in this sheeted layers or striped.

- there are identifiable minerals of quartz, mica, and feldspar.

     i. quartz is quite often found in metamorphic rock. It is one of the most common minerals found in the rocks of the earth. It is usuallu white although in some instances it is rose colored or purple, grayish brown or yellow. 

    ii. mica is made of both muscovite and biotite. muscovite is quite thin and is sometimes called potash mica (it is used in insulation) biotite is a darker color gray because of higher iron and magnesium content. 

    iii. feldspar is often found in metamorphic rock. it is also a very common rock found in over 60 % of the earths crust. Visually you can identify feldspar by its luster and cleavage. it is usually light colored with a range from light gray through pink into reddish and greenish. the luster of this rock is much like porcelain is and the cleavage meaning its angualrity is usually 90 degrees and rectangular.

Geological rock cleavage refers to the tendency of certain rocks to split or break along specific, flat planes of weakness, often due to the alignment of mineral grains or the presence of foliated structures. This phenomenon is commonly observed in metamorphic rocks, such as slate, where the minerals have been subjected to pressure and heat, causing them to reorient and align in parallel layers. The planes of cleavage form because the mineral bonds are weaker in certain directions, allowing the rock to fracture more easily along these planes. Cleavage is an important characteristic in geology as it helps in identifying rock types and understanding the tectonic forces that have influenced the rock's formation and structure.

Gneiss in Texas, like elsewhere, is characterized by its foliated (banded) texture and a coarse-grained appearance. This metamorphic rock is formed under high temperature and pressure, typically resulting in alternating light and dark bands due to mineral alignment. Gneiss in Texas, particularly in areas like the Llano Region, often consists of minerals like quartz, feldspar, mica, and hornblende. 

Here's a more detailed look at the characteristics of gneiss, especially in the context of Texas:

1. Foliated Texture:

Gneiss exhibits a distinct banded or foliated texture, meaning its minerals are aligned in distinct layers or bands. 

This banding is caused by the alignment of platy or elongated minerals under high heat and pressure. 

The banding can vary in orientation, from horizontal to steep dips, reflecting the stresses during rock formation. 

2. Coarse-Grained Appearance:

Gneiss is typically coarse-grained, with individual mineral grains visible to the naked eye.

This coarseness contributes to its distinctive visual appearance. 

3. Mineral Composition:

Gneiss in Texas, like elsewhere, is often composed of quartz, feldspar (potassium feldspar and sodium feldspar), and mica (biotite, muscovite). 

Hornblende is also a common mineral in gneiss. 

The specific mineral composition can vary based on the original rock type and the metamorphic conditions. 

4. Formation:

Gneiss is formed through metamorphism, where existing rocks (igneous, sedimentary, or other metamorphic rocks) are altered by heat and pressure. 

High-temperature and high-pressure conditions are required for gneiss formation. 

Gneiss can form from various parent rocks, including granite, gabbro, and shale. 

5. Examples in Texas:

The Llano region is known for its gneiss formations, including the Valley Spring Gneiss. 

Other examples in Texas include Inks Lake State Park and the Lost Creek Gneiss. 

Gneiss in Texas often appears in banded or layered formations, reflecting the original structure of the parent rock. 

As you observe each of the specimens at ground zero please note what you see and answer the following questions;

A First let us know how many examples of Gneiss do you see here? What Period were these rocks formed and how?

B What would you say by viewing these fine examples of gneiss are the higher percentage of mineral represented and what type? What would you say is the lowest percentage of mineral represented?

C Using the diagram identify 2 different types of cleavage and which mineral is associated with it in these examples at ground zero.

D  Are all the examples here the same? Describe them using the above characteristics as your guide.

E PLEASE share a photo of a fixture that is nearby as proof that you and if you are a group of cachers were at GZ. Group answers are welcome for all included in the photos.

Please send the answers to us using the MESSAGING system on geocaching prior to claiming your find

Here are somew of the sources that were used in the research for this earthcache.

https://en.wikipedia.org/wiki/Gneiss

https://www.beg.utexas.edu/publications/precambrian-rocks-southeastern-llano-region-texas