Mineral deficiency can cause acidity. Good, but Clark makes a mistake in that the oyster shell form is nearly unabsorbed by the body, going right through unabsorbed. Better absorbed forms, and also of magnesium (always balance calcium with magnesium) are:
Also, trace minerals are nearly always deficient in modern society, even in those who eat lots of veggies. Spirulina is a good way to get absorbable forms of trace minerals, or try to find a brand of ionic minerals that has lots of the trace minerals in it. Just my 2 cents.