Beyond Blue: Discovering the Fascinating World of Lapis Lazuli
Mineral Matters #365
Mineral Matters #365! A whole year of posts! 😁
Lapis Lazuli
Today, we are going to look at Lapis lazuli. I watched a YouTube video from the crew at BBC Earth Science about this substance, and its use from early days as a strong blue paint pigment.
You can watch the video here.
Uncovering the Source of Lapis Lazuli's Sulphur Smell
Lapis lazuli is a blue rock, not a single mineral. It is mostly comprised of the mineral lazurite, formula Na7Ca(Al6Si6O24)(SO4)(S3)·H2O, and invariably contains pyrite. It is interesting that the BBC Earth Science guys allowed the comment about sulphur atoms in the lazurite being responsible for the sulphur smell. If you crush a piece of lapis, you can smell sulphur. But that is due to the presence of the pyrite within the rock, rather than the sulphur atoms directly in lazurite.
Why is this?
While lazurite does contain sulphur, these sulphur atoms are part of a complex silicate structure. This strong chemical bonding makes it extremely unlikely that you'd perceive a sulphur odour simply by crushing the rock.
Pyrite, also known as fool's gold, is an iron sulphide mineral with a distinctive metallic lustre. When pyrite is broken or scratched, it can release a faint sulphurous odour due to the breakage of sulphur-iron bonds. This is a common way to identify pyrite in the field.
The Atomic-Level Origins of Lapis Lazuli's Striking Blue Pigment
So why is lapis such an intense blue?
Lazurite contains S−3 anions, which are sulphur ions with a negative charge. These anions absorb certain wavelengths of light, particularly in the red and yellow regions of the visible spectrum. As a result, the transmitted light appears blue to our eyes.
The intensity of the blue colour of lazurite can also be influenced by impurities and the presence of other minerals.
Pigments Explained: The Absorption, Reflection, and Scattering that Creates Colour
A pigment is a coloured material that is used to add colour to a substance or surface. Pigments are typically insoluble crystalline or particulate materials that absorb, reflect, or scatter certain wavelengths of visible light while allowing other wavelengths to pass through.
The use of lapis lazuli as a vibrant blue pigment, dates back to the ancient world, as far back as the 4th Millennium BCE (Before Common Era) in Ancient Egypt.
Lapis lazuli continued to be a highly valued pigment in Byzantine and medieval European art, used in illuminated manuscripts, icons, and panel paintings from the 4th Century CE (Common Era) through the 15th Century.
The Renaissance period (14th to the 17th Century) saw a significant increase in the use of lapis lazuli as an artistic pigment with artists such as Michelangelo, Raphael, Titian, Albrecht Dürer, and later on, the Flemish painter, Jan van Eyck using it.
The increased availability of lapis lazuli during the Renaissance, due to expanded trade routes, allowed these master painters to incorporate the vibrant blue pigment more extensively into their works. The rich, ultramarine blue created by lapis lazuli became a hallmark of many iconic Renaissance paintings, cementing its place as one of the most prized and prestigious pigments of the era.
Painting the Modern World in Blue
In modern times, the primary synthetic pigment that has largely replaced the use of natural lapis lazuli is called ultramarine, a blue pigment that was originally developed as a less expensive and more readily available alternative to the precious natural pigment of lapis lazuli.
Ultramarine is a synthetic compound made up of sodium, aluminium, silicon, oxygen and sulphur. It mimics the chemical composition and blue colour of the lazurite mineral found in lapis lazuli.
Ultramarine is more affordable, highly stable, and available in a wider range of blue hues compared to natural lapis lazuli. This makes it an attractive option for artists, manufacturers and industries. It is widely used today in paints, inks, plastics, cosmetics, and other products requiring a vibrant blue colour. It has largely supplanted the use of lapis lazuli as an artistic pigment.
Below: Lazurite and sodalite (yellow), Afghanistan 48x24x12mm. Photographed under longwave UV light.
