From David Garnett
In “Unsung elements” (18 June, p 36) James Mitchell Crow states they “make up a few parts per billion of Earth’s crust”. While correct for one of the nine – tellurium is about 1 part per billion (ppb) – he is way out with the others.
With reasonable confidence, their crustal abundances are: indium 200 ppb; terbium 1000 ppb; europium 2000 ppb; dysprosium 6000 ppb; yttrium 30,000 ppb; neodymium 35,000 ppb; lanthanum 35,000 ppb; and cerium 60,000 ppb.
Darwin, Northern Territories, Australia
