Worldwide, the Andalusian crystal is considered one of the precious stones most sought after by collectors, and people who believe they have energetic and metaphysical powers as will be seen later.
This article shows all the properties, characteristics, meaning and uses that the andalusite ore.
What is andalusite?
The andalusite It is a mineral that belongs to the group of nesosilicates, it has a chemical formula (SiO4)AlAlO, where 63.2% of the volume is AlO and 36.8% is SiO2, its origin results from regional or contact metamorphism and its use is due to its ability to withstand high temperatures.
It is basically a high hardness aluminum silicate, and it is the polymorph of kyanite and siliminatia.
It also comes in different varieties, the best known is the so-called stone of the cross with its scientific name chiastolite.
Chiastolite variety «Piedra de cruz»
The ore Chiastolite is a variety of andalusite that basically contains black graphite particles that are arranged or distributed geometrically.
Graphite takes its place due to crystal growth within a rock that is undergoing metamorphism, as growth occurs graphite particles become concentrated at interfaces in the mineral crystal.
The result is usually that the graphite forms a cross-like pattern within the mineral, similar to a cross stone.
This means that this mineral has represented a religious or spiritual meaning for centuries, which is why these stones are polished and shaped as amulets and religious gems.
The chiastolite has the same chemical composition and is of similar origin to the andalusite, however the chiastolite will come from rocks with a higher organic content, so that the graphite that is in the crystal is formed.
Characteristics and physical properties
| andalusite | Characteristics and physical properties |
|---|---|
| class, group | Nesosilicate (Aluminum Silicate) |
| Chemical formula | (YesO4)AlAlO |
| Chemical composition | AlO (63.2%), SiO2(36.8%) |
| cleavage | Okay |
| Hardness | From 6.5 to 7.5. |
| Density or specific weight | 3.16 to 3.20 g/cm3. |
| Brightness | Vitreous. |
| Color | Flesh red, reddish brown, pink, olive green. The type of chiastolite variety has carbonaceous inclusions of a dark color. |
| Stripe | White |
| Fracture | Regular. |
| crystalline system | Rhombic. |
| polymorphs | sillimanite and kyanite |
| Origin and formation | It originates from the metamorphism of aluminum slates and schists as a result of regional metamorphism or contact metamorphism. |
| associated rocks | Cordierite in hornfels, granite and granitic pegmatite. |
| Associated minerals | Diaspore, pyrite, pyrophyllite, quartz |
| Applications | Engine spark plugs, refractory porcelains, high-quality crystals serve as precious stones. |
The mineralandalusite It is transparent to translucent and can present a strong pleochroism (green in one direction and red in the other), on the other hand, in some cases there are dark-toned carbonaceous inclusions known as chiastolites.
This mineral is found from red, reddish brown, pink and olive green and even white to gray.
Regarding its physical properties, the andalusite, is characterized by having a density between (3.16 to 3.20 g/cm3), the Mohs hardness is between 7 to 7.5, its brightness is mainly vitreous.
In addition, the color of its stripe is white, it presents a good cleavage and a regular fracture.
The andalusite It has the property of withstanding high temperatures without any type of alteration, which is why it is very useful for the manufacture of ceramics and refractories.
Meaning of the Andalusian stone
Since ancient times, the Andalusian stone has been related to clarity and openness of the mind, that is to say, it is believed to promote a clear vision of the problems that occur in the social environment.
It is also believed that it can help you to better clarify your ideas and make better decisions.
It also signifies achievement and personal development, as it has been associated with personal empowerment and self-esteem. Basically it is believed to help build perseverance and consistency.
It also has a meaning of emotional stability, it is believed that the andalusite stone manages to balance energy and inner chakras. For this reason, many people seek it for meditation and achieve sleep balance, especially very busy people.
Thin-sheet andalusite (optical properties)
The optical properties of andalusite indicate that:
- SHAPE: Generally automorphous. Frequent inclusions of quartz and other minerals
- COLOR: Colorless or pale pink (very rare with green color)
- PLEOCHROISM: Pink-colorless
- RELIEF: Medium +
- birefringence:low
- INTERFERENCE COLOUR:First order gray or white
- EXTINGUISHING ANGLE: 0º = straight
- OPTICAL SIGN:Biaxic (-)
It is common to find this mineral in metamorphic rocks forming the porphyroblastic texture.
It is commonly associated with pyrophyllite, but the two minerals are rarely in textural equilibrium.
In high-temperature assemblages, andalusite can coexist with corundum: in some cases, andalusite is produced with minerals more typical of potash alteration (sericite, potash feldspar, and biotite).
Origin, formation and geological environment
The andalusite It originates during regional metamorphism of schists or gneiss where converging plate boundaries expose rock to temperatures and pressures necessary for its formation.
It can also originate due to contact metamorphism in clayey rocks or as crystals resulting from contact metamorphism of intrusive rocks.
This mineral is usually found in rocks such as schists, micaceous slates and gneiss.
The andalusite It is formed in a certain pressure and temperature range, the pressure in which it is formed ranges from 0 to 4 Kbar and at a temperature greater than 200 °C and less than 800 °C
minerals like andalusite, sillimanite and kyanite have the same chemical composition, but different crystalline structure, this is due to the different conditions of pressure and temperature to which they were subjected, this is called polymorphism. The following image represents the conditions in which these minerals are formed.
The type of information provided by the phase diagram is of great help to the geologist, since if he finds the mineral in the field andalusite it can be related to what conditions of pressure and temperature the rocks were subjected to.
Andalusite uses
Uses in jewelry of the andalusite stone
Due to its great resistance to wear, its hardness and its beauty, especially when it appears in the chiastolite variety, it is almost always used in high-end jewelry, as a gem to make bracelets, pendants, chains, earrings, necklaces, etc.
Geological uses of andalusite mineral
It is known by geologists that the mineral andalusite is almost always associated with regional metamorphism, so it serves to correctly identify the metamorphic rocks associated with this process.
Other uses
- Manufacture at high temperature of porcelain for candles and porcelains of a refractory nature.
- Manufacture of ceramics that withstand high temperatures to be used in furnaces or incinerators.
- As precious stones use those crystals of high quality.
How to identify the andalusite crystal?
The shape of the andalusite Possessing prismatic crystals with a square cross section is one of the main characteristics for its identification, since only a few minerals have this characteristic.
Another characteristic is the strong pleochroism that this mineral presents, when observed to the andalusite in different directions, it can present different colors, the advantage of this pre-ochroic characteristic is the cut that is made at the Andalusian to get gems.
It is basically identified by the physical properties that each mineral presents, in the case of Andalusian it is its vitreous luster, the color of the white stripe and Regular fracture.
In addition, it should be mentioned that the mineral presents a variety of colors, it is commonly found in pink to red, but can occur in colors such as reddish brown, olive green and white to gray.
It has a medium hardness in reference to the Mohs scale and varies between 6.5 and 7.5.