That’ Gneiss Stone: Gneis
I had recently settled on becoming a geology major, which was an outlandish choice in contrast to the long line of engineering majors in the family. My dad pointed to the nearest boulder along the trail.
The boulder was a swirled blend of white, pink, and sparkly grey. My dad is a pretty imposing figure at times, and takes great pleasure in wry teasing of his offspring. Geologically speaking, granite and gneiss are similar, so lumping them together is a reasonable thing to do. In simplified terms, you can think of gneiss as a metamorphic version of granite.Both gneiss and granite are made of feldspars, quartz, mica, and smaller amounts of dark colored minerals like hornblende. Both have tightly interlocking minerals, so they are minimally porous.
On paper, they have similar properties and can be used in the same ways.
The difference between granite and gneiss is in their overall texture and movement. It formed from liquid magma that cooled and crystallized. Granite is like rocky road ice cream – a solidified conglomeration of different ingredients.
Gneiss, arguably, is more visually interesting. It’s characterized by stripes, linear bands, or flowy rivers of color. This pattern is called foliation and it’s a result of the rock being squeezed and heated. Gneiss is like ribbon candy – it’s been folded and swirled while hot, and then left to harden. The stone captures the expressiveness and movement that come from its dynamic origins.
The striped, wavy look of gneiss comes from extreme amounts of compression. The random orientation of minerals you see in granite is an inefficient use of space, sort of like the ragtag pile of magazines you left next to the couch. They align themselves in the same direction when they get buried a few miles deep and pressed between colliding continents.
If the compression involves folding or twisting, as it often does in geologic crumple zones, then you get a stone with wavy or ribbonlike texture. The parent rock of gneiss can be granite, but it can also be shale or an impure sandstone (meaning it contains more than just pure quartz sand). Previous articles have described the continuum of metamorphism as a stone is exposed to increasingly torturous heating and compression. Shale becomes slate , then phyllite, then schist, then gneiss.
Each of these steps is gradual, as the stone slowly changes in response to the conditions it experiences.
In gneiss, you can often see fluid-looking pockets of quartz that were melted while the rest of the rock remained solid. It forms the “basement rocks” that are below most land masses.
Newer stones pile up on top of the basement. But sometimes those newer layers get scraped away by erosion, and the older layers get heaved upward by mountain building. Behold, the basement sticks out at the surface. Gneiss is no exception; it comes in many variations, and can appeal to many different styles and tastes.
While all gneiss is striped or banded, the bands can be straight, gently wavy, or chaotic. The colors can be mostly dark, or mostly light. The stone can be black and white, or black and pink, or black and gold, or nearly any combination thereof. Interesting minerals like garnets (dark pink, round specks) or kyanite (blue, elongated crystals) can liven up the usual combo of minerals.
Note that a stone of a given name can have different patterns depending on which direction it is cut and which part of the quarry it is from.
When shopping for gneiss, it’s helpful to select specific slabs to get the look you want. It tends to be blocky and dense, and makes robust dimension stone for buildings, walls, and landscaping. Gneiss is also a durable choice for interior uses, and makes a fantastic countertop or tile. The minerals in gneiss do not etch when exposed to normal household acids like vinegar or citrus.
It tends to be low in porosity, meaning it is not likely to stain, but there is some variability here, and some gneisses benefit from sealing. There are two caveats when working with gneiss.
One is that the linear grain in the stone can sometimes mean that the stone wants to split along those layers.
This is not common, but worth considering if you have a large overhang. Try to minimize overhanging areas where the overhang is parallel with the natural grain of the stone. Or, if you do use the stone this way, be sure it’s well-supported underneath. Second, keep an eye out for large areas of mica minerals, which are readily identified by their glittery look. In gneiss, mica minerals tend to form their own layers, which are likely weaker areas than the surrounding feldspar and quartz. Some of the dark colored gneisses have large amounts of biotite mica. All in all, gneiss is a durable, functional stone with a pleasing range of aesthetics.
If you crave a rock-solid building stone or a countertop with flow and personality, check out a nice piece of gneiss.
Soapstone is remarkably resistant to staining or damage from acids.
The density of the stone makes it practically impervious.
Gneiss displays distinct foliation , representing alternating layers composed of different minerals.
However, unlike slate and schist, gneiss does not preferentially break along planes of foliation because less than 50% of the minerals formed during the metamorphism are aligned in thin layers.
Gneiss is typically associated with major mountain building episodes.
Gneiss Et Granite
During these episodes, sedimentary or felsic igneous rocks are subjected to great pressures and temperatures generated by great depth of burial, proximity to igneous intrusions and the tectonic forces generated during such episodes.
Grain size -medium to coarse grained; can see crystals with the naked eye. Mineralogy – felsic minerals such as feldspar ( orthoclase, plagioclase ) and quartz generally form the light coloured bands; mafic minerals such as biotite , pyroxene ( augite ) and amphibole ( hornblende ) generally form the dark coloured bands; garnet porphyroblasts common. Other features – generally rough to touch. Uses – dimension stone for building facings, paving etc. Granitic gneiss mineral information and data. Certaines parties de la roche fondent et constituent alors le mobilisat ( magma à composition granitique). Attention que la fusion totale d’une metapelite donnera en theorie une roche dioritique, pas granitique. Il y a un type tres rare de roche qui est ignee et contient de la calcite (composant principal des calcaires). Le calcaire est fait de carbonates; les roches ignees sont faites de silicates.Rien que pour ca, elles sont fondamentalement differentes chimiquement (et mineralogiquement). Les secrets des supervolcans sont-ils inscrits dans le granite ? Orthogneiss is gneiss derived from igneous rock (such as granite). Gneiss forms at higher temperatures and pressures than does schist . Gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, etc. The darker bands have relatively more mafic minerals (those containing more magnesium and iron ).
(Granite Biotic Grey) Gneiss Of Ukraine Frotteurism / Nat
Rock Outcropping at Harriman State Park.
In kyanite gneiss, crystals of kyanite appear as random clumps in what is mainly a plagioclase (albite ) matrix .