Essex Rock and Mineral Society
Mineral
colours
by
Stuart Adams
One of the main attractions of a mineral specimen is its colour and is often due to chemical composition.
Commonly transition metals (nickel, chromium, copper etc.) impart the colours which attracts the eye, the mineral in question absorbing other wavelengths of the spectrum and emitting only that you see. For instance the unmistakable blue of Dioptase ( a hydrous silicate of copper) is due to the presence of copper. Malachite ( a mixed copper hydroxide/ carbonate) has a characteristic green and is also due to copper. The atomic structure of this mineral which is totally different from Dioptase illustrates how colour does not always indicate composition.
But what of the colour of Opals. These hydrous silicas contain virtually no trace elements yet display colours ranging from the reds of the Mexican fire opals through to the blues of the Australian opals. Why should a substance of such simple composition displays such attractive hues?
The work ( amongst others) of Jones Sanders and Segnit (1, 2) using an electron microscope and very thin slices of opal found that they were composed of spheres of hydrated silica. The colour is a result of daylight being diffracted by the spheres and the wavelength observed depends on their size. Where they are similar size spheres then the colour is uniform and consequently mixed colours are from mixed sized spheres.
1. Jones J.B. Sanders J.V. and Segnit E.R. (1964). Structure of opals
Nature, 204, 990 - 991.
2. Jones J.B. and Segnit E.R. (1971) The nature of opal. 1. Nomenclature and constituent phases. Journal of the Geological Society of America, 18, 57 -68.
A
good fist sized Opal specimen from Stuart's collection. Looks like someone
has dropped a Neapolitan ice-cream over a piece of rock!
Closer
view of the Opal, you would need a hand lens to see this detail.
Another
specimen of Opal from Stuart's collection, again about the size of a clenched
fist.. This is what is called "Mexican Fire Opal". It is also
called jelly opal.
Another
close-up of the above, this piece has depth and good colour. The size
is about as big as one's thumb tip. It seems easier to understand why
they call this "fire opal" or "jelly opal" with this
view?
(The good is good, the bad is bad and this is as wicked as it gets, will Stuart let me cut it, I think not!)
Article © Stuart Adams 2003. Photographs with comments © Roger Coleman 2003