  We studied narrow $^1$H NMR spectra of Mg(OH)$_2$ and Ca(OH)$_2$ powders at
100$-$355 K and 42$-$59 MHz using pulsed NMR techniques. The Fourier
transformed NMR spectra of the proton free-induction signals show the
superposition of broad and narrow components, which can be assigned to immobile
protons and extrinsic mobile protons, respectively. We found that a narrow
spectrum develops on heating above about $T_c$ = 260 K and widens above a
Larmor frequency of about $f_c$ = 50 MHz for Mg(OH)$_2$. The
temperature-induced NMR spectrum and the characteristic frequency $f_c$ of 50
MHz are the noteworthy features of the nuclear spin fluctuation spectra of the
extrinsic protons.
