How Hot Are Your Rocks? Radioactivity in Uranyl-Activated Fluorescent Minerals

by Chris Clemens

Introduction: Uranyl-activated green fluorescence is one of the most common fluorescent responses seen under UV light. It occurs in a wide variety of minerals, generally due to one of two conditions: 1.) The inclusion of trace amounts of the uranyl ion, (U02)2+, as a contaminant not normally present in the mineral. Hyalite opal (opal-AN), chalcedony, and adamite are excellent examples of minerals that in pure form do not contain the uranyl ion and consequently are non-fluorescent, but in specimens where uranyl is present in trace amounts, bright green fluorescence is the result. 2.) Alternately, some minerals contain the uranyl ion as an integral component of their molecular structure, and some of these, such as autunite, schroeckingerite, and andersonite, show brilliant green fluorescence. The following photograph shows a classic example of uranyl-activated fluorescence:

The presence of trace amounts of uranium, in the form of the uranyl ion, is the cause of the bright green fluorescence seen under short wave UV in this agate from Wyoming.

The vivid green glow of uranyl-activated fluorescence can be beautiful and spectacular, and no collection of fluorescent minerals would be complete without at least a couple of examples. Regardless of whether the uranyl is present as a contaminant, or as a structural constituent, all minerals that show uranyl-activated fluorescence contain the radioactive element uranium to one extent or another. My personal collection contains many such minerals that show uranyl-activated fluorescence, and I thought it would be interesting to conduct a survey to determine the relative levels of radioactivity emitted by the uranium component of some of the more commonly-collected species.

Method: The method was simple. A total of 16 uranyl-activated, green fluorescent mineral specimens were selected from my personal collection for inclusion in this study. The selection included specimens representative of activation by uranyl as a trace contaminant, as well as some which contained uranium as an integral structural component. As a negative control, I also included a specimen that contained green fluorescent willemite, but no uranyl. Radioactivity was measured using a Ludlum Model 3 Survey Meter (Geiger Counter) equipped with a Ludlum Model 44-7 alpha, beta, gamma detector probe. For each specimen, the probe was held at a distance of 1 cm above the surface, and measurements were taken over a period of 60 seconds, and an approximate average of radioactive emission was determined in units of counts per minute (cpm). The following photograph shows the experimental set-up:

The specimens surveyed, and the corresponding measured levels of radioactivity are summarized in the following three tables. The minerals are listed beginning with those that were found to be the most radioactive, and are ranked in order of decreasing radioactivity.

Results: The level of background (environmental) radiation in my home was measured and determined to be approximately 20 cpm. This background count was subtracted from the values reported for each mineral specimen.

As was expected, the minerals that were found to emit the highest levels of radioactivity were those in which uranium is an integral structural component and therefore is present in the largest amounts. The measured radioactivity for this group ranged from a high of 40,000 cpm for autunite from Washington State, down to a low of 800 cpm for an andersonite specimen from Utah. Refer to Table 1, following, for photographs of these minerals and a summary of the findings:

Of the minerals which contained uranyl as a trace contaminant, I separated them into two groups- the first group contains those which emitted levels of radioactivity detectable above background. The second group contains uranyl-activated minerals which did not emit level of radioactivity above my minimum threshold for detection.

Of those that emit detectable levels of radioactivity, a specimen of hyalite opal from Zacatecas, Mexico, was found to be the most radioactive (500 cpm). At the bottom of this list is a specimen of tiffany stone (opalized fluorite with bertrandite) from Utah (90 cpm). Refer to Table 2, following for a summary of the result for this group of specimens:

Interestingly, although showing bright green fluorescence, a hyalite opal from San Louis Potosi, Mexico, and several brightly fluorescent chalcedony specimens did not emit detectable levels of radioactivity. Not surprisingly, the negative control specimen from Sterling Hill, New Jersey, did not emit detectable levels of radioactivity either. Refer to Table 3, following, for a list of the minerals that did not emit measurable radioactivity:

Discussion: These results raise the question of safety for the handling and collection of uranium-containing minerals. Certainly any mineral emitting radioactivity above background should be handled with extra care. Fortunately, uranium-238, the most commonly occurring isotope of uranium, and the isotope most responsible for the uranyl-activated fluorescence seen in these specimens, emits weak gamma rays, and alpha particles (helium nuclei) which are less penetrating than other forms of radiation. This type of radiation does not travel very far through the air. To verify this, I did a quick measurement of the hottest specimen in this survey, the autunite from the Daybreak Mine. Using the survey meter, I determined that at a distance of approximately 1 meter from the specimen, the measured cpm level dropped down to background. Therefore, the primary hazards of keeping any mineral which contains uranium includes ingestion and inhalation of dust, production of radon gas through radioactive decay, and long term exposure to the low level gamma radiation emitted if you spend a lot of time in close proximity to your specimens. Taking common sense measures, these specimens should be reasonably safe as long as you wash your hands after handling; keep them away from food and drink; don't store them in a small, confined, poorly-ventilated space; and don't keep them in a spot close to where people will spend a lot of time. The agate, opal, and chalcedony specimens that emit little to no detectible radioactivity should be the safest to handle. However, the hotter specimens, especially those that tend to readily break down into dust-sized particles, demand to be treated with respect. In all cases, as long as the uranium remains outside of the body, it poses a minimal health risk. In the case of my hotter radioactive specimens, I keep them individually sealed in airtight, plastic containers to minimize the production of radioactive dust.

Conclusion: Uranyl-activation produces some of the most brightly fluorescent minerals, and is dependent on the inclusion of the radioactive element uranium in the mineral. If the uranium is present in trace amounts as a contaminant only, these minerals tend to emit lower or non-detectable levels of radioactivity. When the uranium is present as an integral component of the molecular structure, these minerals contain greater amounts of uranium and consequently emit much higher levels of radioactivity. Similar to fluorescence, in which different specimens of the same mineral type can show tremendous variability in fluorescent response, radioactivity is variable within a given species, and is ultimately dependent on the percentage of uranium contained in the mineral and the amount of the mineral present in the specimen. All uranyl-activated fluorescent minerals emit radioactivity, many at levels detectable above background, but most are safe to handle if proper common sense precautions are taken.

Disclaimer: Although I am a research biologist by profession, I am not a radiation biologist. Therefore the information provided in this blog regarding the safety and handling of radioactive mineral specimens is meant to serve as general guidance only, and is by no means complete or definitive. If you chose to collect and/or handle radioactive minerals you will need to do additional research on your own to determine risk and safety.

Webmaster's Note: Chris shared this blog post to the Facebook FMS Fluorescent Mineral Group where it sparked a lengthy discussion on the topic, and even branched out to discuss Vaseline (uranium) glasses. For more discussion view it here:

Further discussion in our Facebook group brought this chart to light. It illustrates the ionizing radiation a person can receive from various sources. A common concern among fluorescent mineral collectors is radioactivity from our rocks (which glow such pleasing shades of green from the uranyl content). Most are quite harmless. (Chart placed in public domain by the author).

Note: There is no direct conversion between beta CPM (counts per minute - an electron count) and (micro and milli) sieverts shown in the chart below, an account of bodily damage. A very rough estimate is 300cpm equals 10 microsieverts, So Chris' autunite in the charts above testing at 40,000cpm might be around 1,300 microsieverts. I don't think it's that simple, but for a rough guide might be sufficient.


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