A good food source of magnesium contains a substantial amount of magnesium in relation to its calorie content and contributes at least 10 percent of the U.S. Recommended Dietary Allowance (U.S. RDA) for magnesium in a selected serving size. The U.S. RDA for magnesium is 400 milligrams per day. (The U.S. RDA given is for adults, except pregnant or lactating women, and children over 4 years of age.) These are my borates. Borates are an interesting class of minerals - there aren't that many of them but they have silicate-like complexity in their structure - the borate ions form tetrahedra and link much like the silicates. The borates are now considered to be a subclass of the carbonates.