Weird Science

Glowing Stone - Thermoluminescence of Fluorite

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (4/2020):

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Ples M., Świe­cący kamień - ter­mo­lu­mi­ne­scen­cja flu­o­rytu (eng. Glo­wing Stone - Ther­mo­lu­mi­ne­scence of Flu­o­rite), Che­mia w Szkole (eng. Che­mi­stry in School), 4 (2020), Agen­cja AS Józef Szew­czyk, pp. 40-42

Any­one fami­liar with my pre­vious work knows that one of the topics I find par­ti­cu­larly fasci­na­ting is energy trans­for­ma­tions that result in the emis­sion of visi­ble light. I’ve alre­ady writ­ten about che­mi­lu­mi­ne­scence in a variety of sub­stan­ces, inc­lu­ding some rather unu­sual ones like sili­con-orga­nic com­po­unds (such as Wöh­ler's silo­xene), as well as about cry­stal­lo­lu­mi­ne­scence, pho­spho­re­scence, and flu­o­re­scence [1] [2] [3]. Today, we’re adding ano­ther beau­ti­ful natu­ral phe­no­me­non to this col­lec­tion: ther­mo­lu­mi­ne­scence.

An Extra­or­di­nary Cry­stal

Flu­o­rite is a mine­ral, a natu­rally occur­ring cry­stal­line form of cal­cium flu­o­ride CaF2. It's also one of the most abun­dant mine­rals on Earth — you can find it across the globe [4].

Amor­phous cal­cium flu­o­ride is a white, odor­less solid (Photo. 1). Since natu­ral cal­cium flu­o­ride is wide­spread in nature — as in the form of flu­o­rite, the focus of this article — there’s lit­tle need to pro­duce it syn­the­ti­cally on an indu­strial scale. Howe­ver, small quan­ti­ties of pure flu­o­ride can be obta­i­ned by reac­ting hydro­flu­o­ric acid HF with cal­cium car­bo­nate CaCO3.

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Photo. 1 – Cal­cium flu­o­ride

Flu­o­rite cry­stals can come in a variety of colors — color­less, yel­low, green, blue, and more. The spe­ci­men I used, which ori­gi­na­tes from Asia, has a deli­cate pink hue — most cle­arly visi­ble in day­li­ght (Photo. 2).

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Photo. 2 – Flu­o­rite, author's col­lec­tion

This mine­ral forms cubic or octa­he­dral cry­stals, which can some­ti­mes reach con­si­de­ra­ble size. It often forms cry­stal­line coa­tings on other rocks and mine­rals.

Flu­o­rite has nume­rous appli­ca­tions. It’s used as a flux in metal­lurgy, as a source of flu­o­rine and hydro­flu­o­ric acid in the che­mi­cal indu­stry, and in the pro­duc­tion of opti­cal instru­ments. Due to its stri­king appe­a­rance, it’s also popu­lar among col­lec­tors.

This mine­ral exhi­bits clear flu­o­re­scence — when illu­mi­na­ted with ultra­vio­let radia­tion, it emits a pink glow [5]. In fact, the very term "flu­o­re­scence" ori­gi­na­tes from this mine­ral.

Howe­ver, flu­o­rite is also known for ther­mo­lu­mi­ne­scence.

Ther­mo­lu­mi­ne­scence mani­fe­sts as light emis­sion fol­lo­wing the hea­ting of a cry­stal that has pre­vio­u­sly been expo­sed to radia­tion of suf­fi­cient energy.

We won’t stop at the­ory — you can easily obse­rve this phe­no­me­non with your own eyes.

The Expe­ri­ment

You’ll need a flu­o­rite cry­stal — even a small one.

As men­tio­ned ear­lier, before con­duc­ting the expe­ri­ment, the cry­stal must be expo­sed to radia­tion of a spe­ci­fic wave­length. This radia­tion needs to be suf­fi­cien­tly ener­ge­tic — visi­ble light sim­ply won’t do. So where do we get such a source?

Keep in mind that this cry­stal lay buried in the gro­und for mil­lions of years before it was disco­ve­red and une­ar­thed. During this time, it was natu­rally expo­sed to back­gro­und ioni­zing radia­tion, which in most cases is more than eno­ugh to excite the mine­ral.

Pre­pa­ring the expe­ri­ment is sim­ple: place the cry­stal on a hot plate and then gra­du­ally begin hea­ting it (Photo. 3).

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Photo. 3 – Start of hea­ting

It's impor­tant not to place a cold cry­stal on a pre­he­a­ted sur­face, as this would likely cause it to crack due to une­ven ther­mal expan­sion.

Once hea­ted to tem­pe­ra­tu­res above 200°C (392°F), the cry­stal begins to glow noti­ce­a­bly, which is cle­arly visi­ble in a long-expo­sure pho­to­graph (Photo. 4).

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Photo. 4 – Ther­mo­lu­mi­ne­scence of flu­o­rite (ISO­800, 10s)


video: sup­ple­men­tary mate­rial

The obse­rved phe­no­me­non lasts for some time — its dura­tion depends on the spe­ci­fic sam­ple — after which the glow fades, and fur­ther hea­ting no lon­ger cau­ses light emis­sion. Of course, if the cry­stal is hea­ted to high eno­ugh tem­pe­ra­tu­res, it will begin to glow due to ther­mal emis­sion, but that’s a dif­fe­rent pro­cess.

Expla­na­tion

So what exac­tly is ther­mo­lu­mi­ne­scence? One might assume it’s a sim­ple conver­sion of heat energy into light. But that would be a mista­ken conc­lu­sion — the tem­pe­ra­ture at which ther­mo­lu­mi­ne­scence occurs is too low for ther­mal emis­sion, and besi­des, the glow cea­ses even as hea­ting con­ti­nues [6].

High-energy radia­tion cre­a­tes elec­tro­nic exci­ted sta­tes in cry­stal­line mate­rials. In some mate­rials, these sta­tes remain trap­ped for exten­ded periods by loca­li­zed defects or imper­fec­tions in the lat­tice, disrup­ting the nor­mal inter­mo­le­cu­lar or inte­ra­to­mic inte­rac­tions.

Quan­tum-mecha­ni­cally, these sta­tes are sta­tio­nary sta­tes with no for­mal time depen­dence; howe­ver, they are not ener­ge­ti­cally sta­ble, as vacuum fluc­tu­a­tions are always affec­ting them. Hea­ting the mate­rial ena­bles the trap­ped sta­tes to inte­ract with pho­nons — that is, lat­tice vibra­tions — allo­wing them to rapi­dly decay into lower-energy sta­tes and emit pho­tons in the pro­cess.

The dura­tion and inten­sity of the light emit­ted during ther­mo­lu­mi­ne­scence depend on the num­ber of trap­ped sta­tes, and the­re­fore on the inten­sity and dura­tion (dose) of the exci­ting radia­tion. This prin­ci­ple forms the basis of ther­mo­lu­mi­ne­scence dating (TL) used in archa­e­o­logy [7].

Refe­ren­ces:

All pho­to­gra­phs and illu­stra­tions were cre­a­ted by the author.

The above text inc­lu­des minor edi­to­rial modi­fi­ca­tions com­pa­red to the ver­sion publi­shed in the jour­nal, aimed at sup­ple­men­ting and adap­ting it for online pre­sen­ta­tion.

Marek Ples

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