Matthew Goetz's Honours Thesis Abstract

The Thunder Bay Amethyst Mine Panorama boasts one of the largest occurrences of amethyst in the world with two distinct generations of veining.  These two generations of veins display vastly different colours, with the first generation veins being colourless and the second generation displaying a purple colour.  It is hypothesized within this thesis that the second-generation veins bleached the first generation veins during their deposition.

A series of heating experiments were conducted to determine how amethyst colour changed with respect to heat and time.  Amethyst samples used for experimentation were collected from the main fault and cut into 15 by 9mm slabs.  These samples were examined using a photospectrometer before and after each heating interval.  The results were then graphed by examining the absorbance of light against energy of light; spectral peaks representing colour centers known to cause amethysts purple colour were then fitted.

Samples were heated at temperatures of 350°C, 300°C and 250°C using a tube furnace attached to a digital temperature interface until colour was complete lost.  Samples heated at 350°C and 300°C became bleached quickly at periods of 1 and 3 hours respectively.  Extended heating of samples at 250°C resulted in no colour loss after a time period of 622 hours.  Peak fitted photospectropic  scans were then examined after each subsequent heating period and compared to the scans before colour loss occurred.  These results decreasing Fe⁴⁺ colour centers when samples lost colour and no change in colour centers when colour loss did not occur.

The stability of colour within amethyst at temperatures lower than 260°C can be attributed to a lack of energy required to overcome the activation energy barrier.  It has therefore been determined that the deposition of second-generation veins likely did not cause bleaching of the first generation veins.