APATITE
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).
