Biomedical Engineering Reference
In-Depth Information
The atomic arrangement of calcium orthophosphates is built-
up around a network of orthophosphate (PO
) groups, which
gives stability to the entire structure. The majority of calcium
orthophosphates are sparingly soluble in water; however, all of
them are easily soluble in acids but insoluble in alkaline solutions.
All chemically pure calcium orthophosphates are crystals of white
color and moderate hardness. However, natural minerals of calcium
orthophosphates are always colored due to impurities, the most
widespread of which are ions of Fe, Mn and rare earth elements [32,
33]. Biologically formed calcium orthophosphates are the major
component of all mammalian calcified tissues [34], while the natural
ones are the major raw material to produce phosphorus-containing
fertilizers [35-38].
4
1.2
Geological and Biological Occurrences
Geologically, natural calcium orthophosphates are found in different
regions mostly as deposits of apatites (belong to igneous rocks),
mainly as natural FA or phosphorites (a sedimentary rock) [36-39].
Some types of sedimentary rocks can be formed by weathering
of igneous rocks into smaller particles [40]. Other types of
sedimentary rocks can be composed of minerals precipitated from
the dissolution products of igneous rocks or minerals produced
by biomineralization (Fig. 1.1) [41]. Thus, due to a sedimentary
origin, both a general appearance and a chemical composition of
natural phosphorites vary a lot [42, 43]. It is a common practice
to consider francolite (or carbonate-hydroxyfluorapatite regarded
as its synonym) as the basic phosphorite mineral [39, 44-48]. A
cryptocrystalline (almost amorphous) variety of francolite (partly
of a biological origin) is called collophane (synonyms: collophanit,
collophanita, collophanite, grodnolite, kollophan) [49-51]. It
occurs in natural phosphorites predominantly as fossil bones
and phosphatized microbial pseudomorphs: phosphatic crusts of
chasmolithic biofilms (or microstromatolites) and globular clusters
with intra-particular porosities [52-55]. Natural phosphorites
(therefore, francolite and collophane as well) occur in various
forms, such as nodules, crystals or masses. Occasionally, other
types of natural calcium orthophosphates are found as minerals, for
example clinohydroxylapatite [56], staffelite (synonyms: staffelit,
Search WWH ::




Custom Search