Fluorescence of rocks and minerals

Fluorescence, I’ve always found it fun, intriguing and often rather beautiful. I’d like to take my interest in fluorescence & combine that with my childhood love of collecting rocks (I so remember my Father’s warnings about not filling his car boot up with rocks, next time 😀 ). Sometimes the word fluorescent is misused to casually refer to an item that is extremely vivid & reflective; what is its scientific meaning?

When a substance absorbs incident electromagnet radiation (for example – light) and very shortly afterwards re-emits that absorbed energy as light of a longer wavelength, this is called fluorescence. Fluorescence can be useful for various applications – from medical research to identification of gems. Here we are going to investigate the natural fluorescence of minerals / rocks …

A rhomboid crystal of calcite illuminated by 'white' light (top) and by 254nm UV light (bottom).

Sample 1 – Calcite, a stable form of Calcium carbonate.

Calcite forms a range of beautiful crystals which can be variously coloured by impurities. When fairly pure & viewed in daylight (or with a studio lamp, as photographed here) it appears white, with a variable opacity.

However, if we exclude the daylight and illuminate the calcite with shortwave ultraviolet light (invisible to the human eye); then the calcite appears intensely red. This is due to absorbtion of the 254nm UV light and the subsequent fluorescent emmision of longer wavelength red light; which we and the camera can see.


How does fluorescence happen?

Think back to your chemistry lessons – imagine an atom or molecule. It’s happily sat there minding its own business, electrons busily orbiting the nucleus; when some shortwave UV-light shines upon it. Ping! One of its electrons gets all excited and jumps up to a higher excitation level. This isn’t very stable and so the electron must soon return to its normal ‘ground’ state. When it does so, some energy is released as light, and this we see as fluorescence.

For further info, Wikipedia has a more detailed article.


A group of botryoidal aragonite crystals illuminated by 'white' light (top) and by 254nm UV light (bottom).

Sample 2 – Aragonite, another form of Calcium carbonate.

Normally viewed as a creamy colour, this botryoidal crystal group emits a low blue-greenish fluorescence when viewed under UV-light.

Other aragonite crystals are known to emit a pinkish fluorescence.


A sample of Fluorite from county Durham, UK. Illuminated by 'white' light (top) and by 254nm UV light (bottom).

Sample 3 – Fluorite, Calcium fluoride.

Fluorite, the mineral that gives fluorescence its name. This predominately green sample is from County Durham, UK.

When viewed under the ultraviolet lamp, it fluoresces strongly with a deep blue-violet colour.


Ultraviolet Light – info & warning.

Violet is the shortest wavelength (highest frequency) light that is visible to the human eye. Beyond that lies the invisible ultraviolet, which is often sub-categorised as follows:

UV-A 315 nm to 400 nm example ‘Black lights’ used for effects shows

UV-B 280 nm to 315 nm

UV-C 100 nm to 315 nm example germicidal lamps

Some geological specimens will fluoresce at a wide range of UV wavelengths but most are best illuminated with shortwave UV-C lighting; like the 254 nm lamp I have been using. This comes with risks for the user and relevant protection should be maintained. Do not expose skin to the lamp’s light, do wear protective glasses and never look directly at the lamp.


Nodules of chalcedony illuminated by 'white' light (top) and by 254nm UV light (bottom).

Sample 4 – Chalcedony nodules

Chalcedony, a widely variable, mixed crystalline form of Silicon dioxide.

The rather dull nodules of this specimen transform to a mix of orange & violet when illuminated by shortwave UV-light.


Photographing the fluorescent samples.

Any standard camera should be suitable for photographing these subjects but the following features & accessories would be helpful:

  • macro or close focusing ability
  • long exposure options
  • remote or timed shutter release
  • tripod or other sturdy mount

Your needs will vary dependant upon the size of your rock samples and the strength of your lighting source. I chose to work with a relatively low power UV-C lamp, somewhat reducing cost & safety concerns but this does then necessitate long exposures.

I also built a blackout box to help with the photography. I used some 7 x 1 inch shelving timber to construct a small box with a 1/2 open front for the camera to look through. The inside is painted with matt black acrylic (thanks Tamsin) and the roof has a small slot for the UV-C lamp to rest in. A black gloss ceramic tile is used as a base, this gives a nice reflection when photographing with ‘white’ light. Finally, black material was draped over the entire setup, including the tripod mounted DSLR, thereby further reducing any extraneous light.

Photographic anatomy of a flower

I’ve often felt that the textbook pencil diagrams of a flower’s anatomy are a little lacking in the beauty of a flower. So, when one of our lily flowers had a petal damaged by the wind, I decided to take a macro photograph & label it up for education. As always 🙂 the project ended up going a little further …

Firstly let’s define the different parts labelled in the featured image above (a larger version of the diagram may be viewed or downloaded from the gallery at the bottom of this article) :

    Stamen – The stamen is the male part of the flower. It consists of the Anther & the Filament. The Anther is the pollen bearing part of the flower. Pollen particles develop from microspores within the anther & are then dispersed from the surface of the anther. The Filament supports the anther in whatever position that flower has evolved to require.
    Pistil / Carpel – Is the female part of the flower. A flower may have varied number of Carpels consisting of an ovary joined to one or several Style & Stigma. The Stigma is the area which receives pollen, it is often uppermost & of a folded or branched nature that is sticky on the surface. The Style is the supporting structure that joins the Stigma & Ovary, it is through this tissue that a pollen tube will grow when the flower is fertilised. The Ovary contains the ovules that will be fertilised by pollen, from which union the plant’s seed will ultimately form
    Calyx – is the combined modified leaf structures known as Sepals & Petals. Petals – are formed from modified leaves and surround the reproductive organs mentioned above. They are often used not just for protection but also to advertise the flower to potential pollinators. As such they are frequently decorative & brightly coloured. The Sepals (not shown) are normally plainer, often green. They protect the flower as it forms & often help support the petals when in full bloom.
    Stem – the supportive structure that physically holds the complete flower structure, whilst also containing the structures required to supply nutrient & water to those structures. (Perhaps something to cover another day)

Notes:
1. The flower pictured has lost one of its anthers in the blustery weather, as well as one petal folding over to expose the ovary.
2. You are free to download & use the labelled photograph for educational purposes as long as the credit to me is kept in place.

Please refer to the gallery below for more detailed macrophotography & micrographs of these structures:

The Syrup Tin Experiment

Now for something a little bit different. I don’t know if you ever did this experiment in school, but it was a classic for young science students, back in my day. (Kids, please don’t do this without a responsible adult).

1. Take a finished syrup tin (good excuse for lots of pancake eating), wash the tin out, then place a small amount of water in the tin and put the lid on firmly.

2. Place the tin on a stove / gas burner. Check that there’s nothing to be broken when the lid comes off. Light the stove, stand back & watch.

3. As the contents of the tin heat up, the water will boil & become steam. These actions will steadily raise the pressure within the tin. At some point the internal pressure will overcome the lid’s resistance and the lid will fly off the tin at quite a rate – it’s been given some Kinetic Energy.

So I initially thought it would be fun to recreate this childhood experiment & perhaps film it at a moderately high framerate. But then curiosity got to me and I wondered if we could estimate the speed at which the tin lid moves; perhaps we could even estimate it’s energy level.

Below is the edited & narrated video produced from the Nikon B700 @ 120fps and the GoPro @ 240fps:

So now we need to collect together our measurements & do some calculations:
—–
Distance (d) from top of syrup tin to top of camera frame = 17cm=0.17m (measured from centre point of lid)
Frame Rate of Camera = 240 fps
Number of frames for tin to leave frame view = 2 frames (As carefully viewed frame by frame; we were lucky with the timing of the lid pop).
—–
Time taken (t) = 1 / (frame rate / no. of frames) = 1/120s = 0.0083s

So using the simple formula for speed: s=d/t
s=0.17/0.0083=20.4m/s
Call it 20 metres per second. In imperial units, that’s about 45 mile per hour.
—–
Kinetic Energy=0.5mv2 (expressed in kg & m/s)

Mass of syrup tin lid = 8g

So 0.5*0.008*202=1.6 Joules of Energy

That’s in the same order of energy as a farm electric fence pulse, so don’t get hit by that flying lid, (our fence line runs a minimum of 3 Joules).
—–
For comparison a high velocity bullet for a small calibre rifle (.22) might have a mass of 2g but a muzzle velocity of 500m/s. That extra velocity packs a big punch because we’re talking v2, so:

0.5*0.002*5002=250 Joules of Energy
—–

So in summary, we did manage to estimate the lid velocity at ~ 45mph and also its energy level of ~ 1.6 Joules. Enough to leave a decent splatter on the studio ceiling!