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A common mineral of barium is barytes, or barium sulfate ($\ce{BaSO4}$). Because elements in the same periodic group have similar chemical properties, we might expect to and some radium sulfate ($\ce{RaSO4}$) mixed with barytes since radium is the last member of Group 2A. However, the only source of radium compounds in nature is in uranium minerals. Why is it so? I searched on the internet but couldn't find useful insights

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    $\begingroup$ The longest lived isotope of radium has a half life of 1600 years. So you look in uranium ores since radium is part of the decay chain from uranium to lead. $\endgroup$ – Jon Custer Jun 16 at 0:21
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    $\begingroup$ Very slightly adding to what Jon Custer commented, just look up radium in wikipedia: the decay chain for that longest lived radium isotope is right there. $\endgroup$ – Ed V Jun 16 at 0:30
  • $\begingroup$ My guess is because the radium ions are too large to fit into the crystal lattice of the mineral. So the radium is not incorporated into the baryte mineral. $\endgroup$ – ĐỨc Lê Hồng Jun 16 at 0:51
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    $\begingroup$ @ĐỨcLêHồng No. The radium is formed in situ in uranium and thorium minerals. There is no “separate” radium available for mineral formation on typical geologic time scales. $\endgroup$ – Ed V Jun 16 at 0:55
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    $\begingroup$ @DucLeHong. Even if some radium would have been incorporated into some barium substance, this phenomena must have happened more than one million years ago. And meanwhile, this radium would have disappeared by natural radioactive decay. $\endgroup$ – Maurice Jun 16 at 9:28
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You can't find primordial radium because it's half life is too small compared to earths age.

Even the radium isotope with the longest half life, $\ce{^{226}Ra}$ has a half life of only 1600 years which is magnitudes smaller than the age of the earth, which is estimated to be around $4.54\times 10^9$ years[1].

This means that whatever radium we can find in today's age must be regenerating from a radioactive parent source. The most commonly known of those sources is as an intermediate in the radioactive decay of uranium and thorium to lead. From The Radiochemistry of Radium[2]:

Isotopes $\ce{^{223}Ra}$, $\ce{^{224}Ra}$, $\ce{^{226}Ra}$ and $\ce{^{228}Ra}$ are part of the decay chains of natural thorium and uranium isotopes; since thorium and uranium have very long half-lives, these daughters are continually being regenerated by their decay.

References:

[1]: Dalrymple, G. B. The Age of the Earth in the Twentieth Century: A Problem (Mostly) Solved. Geol. Soc. Spec. Publ. 2001, 190 (1), 205–221.

[2]: Salutsky, M. L.; Kirby, H. W. The Radiochemistry of Radium; Mound Lab., Miamisburg, Ohio (Grace (W.R.) and Co., Clarksville, Md. Washington Research Center), 1964.

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