In this article, you’ll get to know some commonly known hydrothermal rocks, as well as information about hydrothermal rocks to better understand them.
Hydrothermal Rocks Examples (Characteristics and Types)
What are hydrothermal rocks?
When referring to hydrothermal rocks, we are more accurately talking about the hydrothermal mineral veins and deposits that are formed through hydrothermal activity that takes place deep in the earth and are then either discharged through thermal vents like geysers and hot springs, or crystallized into veins as the hot water passes through fractures in the host material.
The temperature of the water passing through the crack and the pressure at which it does so determines how the mineral crystals shape and how they will be oriented inside the rock.
Hydrothermal minerals differ from metamorphic rocks as a result of this process.
The solid-state transformation that metamorphic rocks go through creates very different minerals from those that form when water or gases heated by magmatic sources pass through these fractured materials.
Much of the water in the hydrothermal process is seawater flowing down through the oceanic crust.
Rain water or snow melt that seeps through the soil to bedrock (referred to as meteoric water), can also be the source of hydrothermal transformation when it becomes heated.
As the water passes through the crust, deep in the earth, its temperature rises significantly, and eventually, the water is forced back to the surface, cooling as it ascends.
This heating and cooling process as it passes through various layers of the earth causes chemical changes to occur in minerals that are being carried by the water as well as those that the water comes in contact with.
The result is various hydrothermal minerals.
Many of these hydrothermal minerals form in layers similar to how sedimentary rocks form their layers.
There is a name for the process through which hydrothermal waters rich in ions pass through rock fractures and porous spaces, dissolving part of the original rock’s softer material (like limestone, calcium and magnesium) and depositing these new elements among the existing minerals in the stone.
It is referred to as metasomatism and it changes the chemistry of the original stone as it leaves behind new elements such as sodium, silica, iron, oxygen and potassium.
The most common hydrothermal mineral to find in rock veins is quartz.
Calcite is fairly common as well.
If enough space is present in a fracture, some gemstones can form in hydrothermal rocks such as emeralds, topaz, tanzanite and garnets.
What are some examples of hydrothermal rocks or minerals?
Cinnabar
Cinnabar forms when hot water or vapors are pushed up through the earth by volcanic activity and is found in rocks at a relatively shallow depth where the temperature is below 200℃ or 392℉.
Cinnabar contains toxic mercury sulphide which makes it a hazardous material to handle, which means it is unusable for things like jewellery, although before people knew it was toxic, it was used for exactly that because of its bright red hues.
It was also previously used as pigmentation in vermillion paint and red lacquer in China.
These days it is primarily mined for its mercury content, which is extracted through heating and trapping the vapors of liquid mercury.
Mercury has had many uses over the years, most of which have been discontinued as the mercury is replaced with nontoxic alternatives.
It was widely used for thermometers, barometers and gravity switches because of its conductive properties.
Identifying cinnabar in the wild is fairly easy due to its striking red tone.
Always be careful when you discover red minerals near where recent volcanic activity is visible.
The most common type of cinnabar found are the dull and impure ore chunks.
The vibrant red crystals are much more rare.
While cinnabar is quite soft on the Mohs scale, coming in at 2 – 2.5, it is an extremely heavy non-metallic material.
Cinnabar samples should be clearly and properly labelled to warn people of its toxicity. Never improperly handle the material.
Fluorite
Fluorite is widely used in the metal, ceramic and chemical industries as a component in a number of processing methods for products.
It has a chemical composition known as CaF2 (calcium fluoride).
There are also uses for fluorite in the lapidary and optical industries as well.
Identifying fluorite specimens in the field is fairly easy to do as it has a distinct, four directional cleavage.
No other common mineral has this feature.
While its color isn’t a reliable identifier, the yellow, purple, light blue and green hues are good visual clues that a sample may be fluorite.
As its name suggests, fluorite can appear fluorescent when subjected to ultraviolet light, giving off a blue-violet glow.
In its raw form, if the color is vibrant enough, fluorite can also serve as ornamental gemstones.
Fluorite finds are rather common all over the globe.
It can be present in limestone and dolomite deposits as well as other hydrothermal and carbonate rocks.
Fluorite veins can contain metallic ores ranging from tin and silver, to copper, zinc and more.
Rhodonite
Rhodonite is one of two uncommon minerals made of manganese silicate found in small deposits in metamorphic rocks all over the world.
Rhodonite tends to have other elements like calcium, iron and magnesium present.
The other manganese silicate mineral is called pyroxmangite.
Rhodonite typically has a beautiful pink color but red samples can be found as well, though they are much more uncommon.
Rhodonite used to be mined in some parts of the world for its manganese, but these days the only real use for these rocks is in the lapidary field.
Larger pieces tend to be valued as ornamental decorations.
Tumbled stones are possible, but the manganese oxide present in many rhodonite pieces can make it difficult to polish as the softer material around the manganese oxide rubs away much quicker.
Graphite
Graphite is crystalline carbon that is commonly found in igneous and metamorphic rock deposits.
It breaks under low pressure, is considered to be extremely soft, and very light.
However, it has a very high heat tolerance and is mostly inert.
Graphite’s wide range of properties make it a much sought after material for metallurgy and manufacturing purposes.
Some graphite can also be found in seams of coal that have gone through a hydrothermal metamorphic process, leaving behind mineral graphite.
Serpentine
Serpentine forms when ultramafic rocks (like dunite and peridotite) go through a hydrothermal metamorphic process that changes the mineral structure of olivine and pyroxene into serpentine minerals.
These rocks are usually near the oceanic crust and upper mantle of the earth, but are rarely on the earth’s surface, with the exception of an exposed oceanic plate that reveals the serpentinite rich layer at the surface.
Exposed areas like this are referred to as ophiolites and these are laden with valuable materials that otherwise would not be easily accessible.
Minerals found in these layers include jade, chromite, serpentine and magnetite.
Serpentine looks very similar to jade in color and is often mistaken for the latter, much to the frustration of many geologists.
To tell serpentine apart from jade, look for the snakeskin patterned appearance and slippery feeling that gives it its name.
It has a waxy appearance, is easily polishable and carvable.
It is this last property that makes it popular as a sculpting material for statues, jewelry and ornamental stone work.
