Aragonite Mineral
The stable type of calcium carbonate under high pressure is the aragonite, which is a widespread mineral. It can be characterised by its greater hardness and specific gravity from the more typical type of calcium carbonate calcite. Aragonite is often to be found in the oxidising zone of ore minerals, in serpentine and other rocks, in soil and in deposits of iron-ore, at low temperatures close to the surface of the Earth. The mineral Aragonite is usually found in pearls. It is polymorphous with calcite, vaterite and potentially reverses it to calcite in normal conditions over time (the same chemical formula but different crystal structures).
Aragonite is the important mineral in the shells of many marine invertebrates. These animals may secrete the mineral from water which would usually produce limestone; they do this using not entirely understandable physiological mechanisms.
Aragonite is a metamorphous carbonated gemstone found naturally in stalactites or in the hot springs. Due to its small, and branch-like crystal shapes, this type of aragonite is sometimes referred to as floss ferri. The mother-of-pearl lining of the mollusk shells contains a further cause of Aragonite. It is separated by the mantle of mollusks in pearl oysters, and builds the nacre layers which the oyster uses to produce the pearl itself. The several thin layers of aragonite add the perl-like lustre and elegance to the abalone shell.
Formation & Colors
Aragonite naturally forms the calcareous endoskeleton of warm and cold-water corals in almost all mollusk shells (Scleractinia). Aragonite tubes are present in some serpulids. Due to the close biological monitoring of mineral deposition in mollusk shells, certain shapes of crystal are different from those of inorganic aragonite. The whole shell of some mollusks is aragonite; in some aragonite forms only distinct sections of a bimineralic shell (aragonite plus calcite). An iridescent material, called ammolite, is made up of the nacreous layer of some extinct ammonite fossil shells.
Aragonite also occurs as inorganic precipitates in the oceans and in caves respectively such as marine cements and speleothems. Aragonite in serpentinites, where high Mg in pore solutions seem to inhibit calcite growth and favour aragonite precipitation, is not unusual.
Aragonite is metastable at the surface of the Earth at low pressures and thus usually substituted in fossils with calcite. Aragonite is largely unknown and older than Carboniferous. It can also be synthesised in water-ethanol mixtures at ambient temperatures by adding a solution of calcium chloride to the sodium carbonate solution at temperatures above 180 °C (140 °F).
The "sputnik" variety from Morocco is the most popular Aragonite on the market. They almost always have an amber hue, golden brown but can also look colourless. Other common color varieties of Aragonite include blue aragonite, pink aragonite, white aragonite, red aragonite, green aragonite, and orange aragonite.
Calcite Aragonite
The composition of calcium carbonate forms as Aragonite as well as Calcite, and the crystallisation of these two minerals only differs. In trigonal crystals, calcite, the more common mineral, forms while aragonite forms orthorhombic crystals. Aragonite and Calcite crystals are sometimes too small to be measured individually and these two minerals can only be distinguished by optical or x-ray tests. Without complex testing it may also be impossible to know the real identity of microcrystalline types of aragonite or calcite, which may also confuse these species.
The greatest Aragonite crystals are the twinned growth of the pseudohexagonal trilling three individual crystals. Even though the orthorhombic system crystallises Aragonite, most prismatic crystals are formed hexagonally due to twinning. Trillings can be distinguished from each individual member crystal through their multi-directional basal striations.
After aragonite, additional minerals may form pseudomorphs. Calcite after Aragonite, which is a pseudomorph after an established paramorph, is a particularity of the mineral universe. Some available aragonite crystals are in fact calcite after aragonite. Copper after Aragonite is an uncommon but common pseudomorph. Sand inclusions which give a sample the brown colour, may also contain aragonite.
As a deposition product from hot mineral-rich springs, an especially interesting type of Aragonite is present. When the water comes out of spring, calcium is released, and mounds and thick crusts are formed around sources. They could be sculpted and called "Onyx Marble" and "California Onyx," if these are banded.
Most organic substances such as pearls and corals are made up primarily of aragonite. Pearl and mother-of-pearl have an iridescent surface that is essentially the layer of aragonite that has mollusks and associated invertebrates. Some types of aragonite, especially Flos Ferri, are fragile and fragile and can break easily when touched. Such samples must be very careful.
Aragonite and calcite, due to the nano and microstructural structures of the general architecture, are the two calcium carbonate polymorphs that give the shell of molluscan bivalves their strength and elasticity. Nacre, or pearl's mother, is not yet classified as osteoinductive. It consists of Aragonite and produces the vertebrate bone.
Physical Properties of Aragonite Crystals
Aragonite's crystal structure behaves like a prism, since it has three triangular sides, or orthorhombic structure. This quality prism, usually called "orient" in pearls, brings light through a diffracted range of gleaming colour. The most notable "orient" of Tahitian pearls is the thousands of aragonite layers that are used to cover the pearl seed. Let's examine all the following properties:
Chemical Formula: CaCO3
Composition: Calcium carbonate, sometimes with some strontium, lead, and zinc.
Variable Formula: (Ca,Sr,Pb,Zn)CO3
Color: Colorless, white, brown, gray, yellow, red, pink, purple, orange, blue, green
Hardness: 3.5 - 4
Crystal System: Orthorhombic
Crystal Forms and Aggregates: Pseudohexagonal trillings, in the form of elongated prismatic crystals or short tabular ones, are the most common crystallised habit. Unusually in individual crystals, untwinned. There are several aggregate types of pseudo hexagonal crystals including acicular, radiating, fibrous, stalactitic, botryoidal, oolitic, tuberose, granular, embedded and ball-like protrusions.
Transparency: Transparent to opaque
Specific Gravity: 2.9 - 3.0
Luster: Vitreous, dull
Cleavage: 3,1 - prismatic ; indiscernible,2
Fracture: Subconchoidal
Tenacity: Brittle
Other ID Marks:
1) May fluoresce blue, pink, yellow, or cream.
2) Clear specimens display a visible double refraction.
Complex Tests: Effervesces in acids, even if cold and diluted.
In Group: Carbonates; Aragonite group
Striking Features: Poor cleavage, twinning habits, strong effervescence, and low hardness
Environment: Sedimentary and evaporite deposits, deposits of hot spring, hydrothermal ore, igneous traprock and metamorphic schist habitats.
Rock Type: Igneous, Sedimentary, Metamorphic
Characteristics of the Mineral
Aragonite is a calcite that can be found mostly in sedimentary rock. Calcite becomes limestone. Aragonite has a similar calcite chemistry, but its composition, symmetry, and various crystal shapes are different. Blue, brown, black, gold, green, grey and white is available in aragonite colour. It is primarily mined in Spain, Bolivia, Mexico, Morocco, Namibia, UK and the USA. Some shapes are very delicate. When struck, Aragonite quickly splits. The mineral also has sand, and some of the minerals are brown.
Aragonite Gemstone
Perls are also regarded as gemstones, but they are not. They are rocks, partially from aragonite. Aragonite is the key ingredient of pearls and coral in particular. The colour of the stones stems from the process that starts in oysters. Perles are made of aragonite on their surface. We have already discussed that argonite forms in different colours such as blue aragonite, pink aragonite, white aragonite, red aragonite, green aragonite, and orange aragonite.
Aragonite Uses
In aquaria, the replication of the reef conditions, aragonite is considered important. Aragonite supplies the essential materials for the long life of the sea and also retains the pH of the water close to its natural level to avoid biogenic calcium carbonate dissolution.
Aragonite has been tested successfully for removal from polluted wastewater pollutants such as zinc, cobalt and lead.
Some water conditioning companies say that their technology transforms calcite to aragonite for limescale reduction.
Conclusion
The stable type of calcium carbonate under high pressure is the aragonite, which is a widespread mineral. Aragonite is often to be found in the oxidising zone of ore minerals, in serpentine and other rocks, in soil and in deposits of iron-ore. It is metastable at the surface of the Earth at low pressures and thus usually substituted in fossils with calcite. The mineral is also found as a metamorphous carbonated gemstone found naturally in stalactites or in the hot springs. It can also be synthesised in water-ethanol mixtures at ambient temperatures.
FAQs on Aragonite
1. What is aragonite and what is its chemical formula?
Aragonite is a carbonate mineral, one of the two common, naturally occurring crystal forms of calcium carbonate. It is a polymorph of calcite, which means it shares the identical chemical formula, CaCO₃, but possesses a different crystal structure. This structural difference is the reason for its distinct physical properties compared to calcite.
2. What are the main uses and importance of aragonite?
Aragonite has several important applications, particularly in environmental and biological contexts. Its main uses include:
- Aquarium Substrate: It is widely used in reef aquariums to replicate natural marine conditions, helping to maintain stable water pH and providing essential calcium for coral growth.
- Wastewater Treatment: Studies have demonstrated its effectiveness in removing heavy metal pollutants such as zinc, cobalt, and lead from contaminated water sources.
- Biogenic Structures: It is fundamentally important as the building block for pearls and coral skeletons, secreted by marine organisms.
3. Where is aragonite typically found in nature?
Aragonite is found in various geological and biological settings. Geologically, it precipitates from marine and freshwater environments, forming deposits in hot springs (as travertine) and caves (as part of speleothems). Biologically, it is a crucial component of the shells of many molluscs and is the primary mineral that constitutes both pearls and the skeletons of most corals. Significant deposits are found in Spain, Italy, Germany, and Mexico.
4. What are the key physical properties of aragonite, such as its hardness?
The key physical properties that define aragonite include:
- Hardness: It has a Mohs hardness ranging from 3.5 to 4, making it slightly harder than calcite but still relatively soft and prone to scratching.
- Crystal System: It has an orthorhombic crystal system, which often results in needle-like (acicular) or twinned hexagonal-shaped crystals.
- Solubility: It is slightly soluble in water, particularly in acidic conditions. This property is why it is not advisable to expose aragonite specimens to water for extended periods.
5. How does aragonite differ from calcite if they have the same chemical formula?
Although both aragonite and calcite are forms of calcium carbonate (CaCO₃), they are known as polymorphs and differ primarily in their crystal structure. Aragonite features an orthorhombic crystal system, whereas calcite has a trigonal system. This fundamental structural difference leads to other variations:
- Stability: Calcite is the more stable form under normal surface conditions. Over geological time, aragonite will naturally convert to calcite.
- Density and Hardness: Aragonite is slightly denser and harder than calcite.
6. Why is aragonite considered metastable and what does this mean for its presence in rocks?
Aragonite is considered metastable at standard surface temperature and pressure. This means it is not in its most stable thermodynamic state; calcite is. As a metastable mineral, aragonite can persist for long durations but will inevitably and irreversibly transform into calcite over geological timescales (tens to hundreds of millions of years) or more rapidly when heated. This transformation is why aragonite is commonly found in younger geological formations but is rare in ancient rocks, where it has already converted to the more stable calcite.
7. How is aragonite formed through both geological and biological processes?
Aragonite forms via two main pathways:
- Geological (Abiotic) Formation: It precipitates directly from water in environments with high concentrations of magnesium or at high pressures, such as in certain metamorphic rocks (blueschists) or as deposits around hot springs.
- Biological (Biogenic) Formation: This is a very common process where living organisms secrete aragonite to build their structures. This is seen in the hard skeletons of reef-building corals, the shells of most molluscs, and the iridescent inner layer of shells known as nacre (mother-of-pearl).
8. What is the importance of aragonite in marine ecosystems and the carbonate cycle?
Aragonite is critically important in marine ecosystems as it is the primary mineral used by reef-building corals to construct their massive skeletons, which provide the foundational structure for entire ocean habitats. Its saturation state in seawater (the "aragonite saturation horizon") is a key indicator of ocean health and the potential impacts of ocean acidification. The formation and dissolution of aragonite are integral parts of the ocean's carbonate cycle, which regulates atmospheric carbon dioxide levels over geological time.
