What is opal and how was it formed?
What is opal?...Opal is Hydrated Silica. Hydrated means that the silica contains water. Silica is also known as silicon dioxide (silicon is one of the most abundant mineral in the Earth's crust). Silicon is a non-metal chemical element that has semiconducting properties. Dioxide tells us that the silicon has two oxygen atoms as part of its make-up. During the Tertiary period (65 million to 2.6 million years ago) in the Australian outback, rocks which now contain opal were subject to significant weathering. Water seeped through sandstone, leaching silica particles and filtering out contaminants until it became trapped by underlying layers of impermeable stone. Over time some of the water dissipated leaving a high concentration of silica particles. Silica
has the natural tendency to try and reduce its contact with water. If the conditions
were conducive and stable, these small silica particles eventually merged together
to create spheres, forming, with the water, what is known as a 'sol'. One important
factor for the play-of-colour to exist is that the spheres must grow to
a size no smaller than 200 and no larger than 350 nanometres (a nanometre [nm]
is one-billionth of a metre), which is exactly half the wavelength size of colour...
400-700 nanometres.
In
time these spheres attract one another but do not merge, and when enough have
come together they sink to the bottom of the solution. These spheres are negatively
charged and give a strong inter-particle repulsion; because of this repulsion
the particles glide around one another after sedimentation (rather like lubricated
ball-bearings in a bucket). The viscosity of the sol determined what size the
spheres would be before settling out of solution. Hence spheres of similar size
tended to settle together.
What is opal?...For there to be colour in opal the silica spheres need to be the same size and grouped together in an orderly three-dimensional array. Because of this uniformity in size, the spheres form tiny regular sized pits which are able to affect the wavelength of light.
What
is opal?...Opal starts out as a liquid then turns into a gel and then eventually
hardens. For this process to occur,enough water needs to dissipate so that
the structure will stay in place. The water content in opal varies between
2% and 20% (it is usually 6 to 10% in precious opal).
Pattern Stacking faults in the silica sphere structure of opals produce many distinct colour patches and determine the pattern of an opal's play-of-colour. Now that you know what opal is, click here to discover why these opal spheres are able to create such wonderful colours and vibrant intensity. Discover some amazing Boulder Opal Jewellery - Here
|
