Is amazonite radioactive
Amazonite is green microcline, KAlSi3O8, 14.05% potassium. Natural potassium is 0.0117% potassium-40, so it is weakly radioactive, as all potassium feldspar is.
Weakly, yes, and not because of its colour. Amazonite is green microcline, KAlSi3O8, 14.05% potassium by mass, and 0.0117% of natural potassium atoms are radioactive potassium-40. Worked through, that is about 4.46 decays per second per gram, or 446 for a 100 g specimen: an activity, not a dose, and shared by every potassium feldspar.
In short
- The honest answer is yes, weakly, and orthoclase, sanidine and every other potassium feldspar are too. The radioactivity is potassium-40, which is a fixed fraction of all natural potassium. Amazonite has it because it is microcline, not because it is green.
- Worked calculation: about 4.46 becquerels per gram for ideal KAlSi3O8, about 446 for a 100 g specimen. Natural potassium works out at 31.7 decays per second per gram of potassium, and microcline is 14.05% potassium. The Handbook's own microcline analysis carries a little less potassium, giving 4.23 per gram.
- The Handbook of Mineralogy does not call microcline radioactive, and that is informative. Of the 176 sheets in this site's reference set, 14 have potassium in their formula and none uses the word. The word appears on six sheets, and every one is a uranium mineral.
- This page does not assert why amazonite is green. You will read explanations of the green elsewhere; the primary sheet this site works from records green among microcline's colours and gives no cause, so neither will we.
- Nothing here converts becquerels into a dose or a health statement. Activity counts decays; what that means for a person depends on things a mineral page cannot know. For the minerals where radioactivity is the point, see the radioactive-minerals guide; for anything personal, ask a Radiation Protection Adviser.
| Material | Formula or source | Potassium by mass | Becquerels per gram | Becquerels per 100 g |
|---|---|---|---|---|
| Microcline, ideal (amazonite) | KAlSi3O8 | 14.05% | 4.46 | 446 |
| Microcline, Handbook analysis | Norra Kärr, Sweden: K2O 16.07% | 13.34% | 4.23 | 423 |
| Orthoclase, ideal | KAlSi3O8, the same formula | 14.05% | 4.46 | 446 |
| Orthoclase, Handbook analysis | Himalaya mine, California: K2O 14.76% | 12.25% | 3.89 | 389 |
| Muscovite, ideal | KAl2(AlSi3O10)(OH)2 | 9.82% | 3.11 | 311 |
| Sylvite, ideal | KCl | 52.45% | 16.63 | 1,663 |
| Albite, ideal | NaAlSi3O8 | 0% | 0 from potassium | 0 |
| Autunite, torbernite, uraninite | Uranium minerals | Not the source | Not calculated here | Their sheets print Radioactive |
Is amazonite radioactive? The worked potassium-40 calculation
Inputs, each from a named source. Standard atomic weights: potassium 39.0983, aluminium 26.9815, silicon 28.085, oxygen 15.999. Potassium-40 is 0.000117 of natural potassium atoms, from the NIST isotopic composition table. Its half-life is 1.248 × 109 years, from the IAEA Nuclear Data Section, which gives it as 3.938 × 1016 seconds.
Step 1, potassium in microcline. KAlSi3O8 has a formula mass of 278.33, of which potassium is 39.10: 14.05% potassium by mass. The Handbook of Mineralogy sheet for orthoclase, the same formula, prints the ideal K2O as 16.92%, which is the same figure expressed as oxide.
Step 2, potassium-40 atoms per gram of potassium. 6.022 × 1023 ÷ 39.0983 × 0.000117 = 1.80 × 1018 atoms.
Step 3, decay constant. ln 2 ÷ 3.938 × 1016 s = 1.76 × 10−17 per second.
Step 4, activity. Multiply the two: 31.7 becquerels per gram of potassium, and times 0.1405 for microcline, 4.46 becquerels per gram. A 100 g amazonite is about 446 decays a second on this ideal formula. Substitute your own specimen's mass; the arithmetic is linear.
Step 5, the real-world correction. Natural microcline is rarely pure KAlSi3O8. The Handbook's analysis, from Norra Kärr in Sweden, gives K2O 16.07%, which is 13.34% potassium and 4.23 becquerels per gram. The Handbook also notes that microcline commonly exhibits banded perthitic intergrowths of exsolved albite, which has no potassium in its ideal formula, so a real specimen sits somewhat below the ideal figure.
Why the Handbook never calls a feldspar radioactive
We checked the whole reference set, because the absence is the useful part. Of the 176 Handbook sheets, searched by script after pdftotext conversion, 14 carry potassium in the formula line — alunite, barrerite, biotite, fluorapophyllite, hydroxyapophyllite, lepidolite, microcline, muscovite, orthoclase, pharmacosiderite, phlogopite, polylithionite, sanidine and sylvite. Not one of the 14 uses the word radioactive.
The word does appear on six sheets — autunite, bassetite, meta-autunite, saléeite, torbernite and uraninite. Every one of the six is a uranium mineral. So in the source collectors actually use, radioactive marks uranium minerals, and potassium-40, which is present in every potassium mineral at the same fixed fraction, is treated as unremarkable.
That is a reasonable editorial choice and it explains the search. The potassium-40 in amazonite is the same potassium-40 that is in orthoclase, which has the same formula; what draws the question to amazonite is the colour. This site's microcline page covers the species, and the minerals where radioactivity is genuinely the subject are on is my mineral radioactive and how to store radioactive mineral specimens.
What the colour does and does not tell you
The Handbook of Mineralogy records microcline's colour as white, pale cream-yellow; red, green, blue and offers no cause for the green. Other sources offer explanations. This page has not verified any of them at a primary source, so it does not assert one, which is the same position the microcline page takes.
What can be said without it: the potassium-40 figure does not depend on colour. A white microcline, a pink orthoclase and a blue-green amazonite of the same potassium content give the same calculated activity. If the question behind the search is whether the green means the stone is more radioactive than other feldspar, nothing in the calculation above says so, and nothing in the Handbook says so either.
The honest cost of finding out more is a measurement, not a reading from a hobby counter: a Geiger–Müller tube tells you something is emitting, not how much of which isotope, as the radioactive-minerals guide explains. For most collectors the calculation above is the more useful answer. The rest of this section is indexed at the collecting guides. Nothing here is offered for sale; the wanted list is the only commercial route on this site.