Apatite is the most common & important phosphate mineral. Apatite is actually a group of specific minerals that vary in chemistry. At its simplest, apatite is a calcium phosphate, but it tends to have lots of elements mixed in (it’s a “garbage can mineral”). The general formula given for apatite is often Ca5(PO4)3(F,Cl,OH) - calcium fluoro-chloro-hydroxyphosphate. Lead, yttrium, manganese, strontium, and other elements can partially substitute in the calcium position. Sulfate, arsenate, vanadate, silicate, and carbonate can partially substitute in the phosphate position. Apatites with F are the fluorapatites. Apatites with Cl are the chlorapatites.
Apatite has a nonmetallic luster, can be any color (mottled greens, yellows, and browns are common), has a white streak, is moderately hard (H≡5), and has hexagonal crystals. Apatite occurs in many igneous rocks, typically granites, pegmatites, and hydrothermal veins. Calcium phosphate is also common in some sedimentary rocks, especially phosphorites, high-phosphate limestones, and bone beds. Apatites are the main components in vertebrate bone and teeth (specifically chlorapatites and hydroxylapatites - they easily convert to fluorapatite upon fossilization/diagenesis).
Apatite (above & below) from Ontario, Canada.
Above: side views of large hexagonal crystals (left: 2.8 cm across; right: 3.4 cm across).
Below: top view of large hexagonal crystal (4.0 cm across).