Amorphous Calcium (Ortho) Phosphates - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 158

with two- or three-dimensional order, then a crystalline (at least, a
semi-crystalline) solid will be formed. Furthermore, in many cases
amorphous materials can be produced by additives, which interfere
with the ability of the primary constituent to crystallize. For example,
addition of soda to melted silicon dioxide results in amorphous
window glass and addition of glycols to water results in a vitrified
solid [83]. More to the point, amorphization of many solids might
be achieved by applying mechanical forces, e.g., by intensive milling
[93, 94], as well as by irradiation [95, 96].
From a thermodynamic point of view, amorphous materials are
at best metastable. Given a sufficient time, they tend to transform
to crystalline phases that are thermodynamically more stable.
Interestingly, but when an unstable crystalline solid is transformed to
an amorphous phase, this transformation frequently exhibits features
that are associated with ordinary melting. Namely, amorphization
frequently begins at grain boundaries, surfaces or other defect sites,
as does ordinary melting. Further, as the transformation proceeds,
a sharp interface that separates the amorphous materials from the
untransformed crystalline material is always observed [82].

2.3 Amorphous Calcium Phosphates (ACPs)

2.3.1 History

According to Eanes [20], the history of ACPs looks as this: “In 1955,
Robinson and Watson [97] were the first to suggest that a substantial
portion of newly formed mineral in young bone was not crystalline.
Instead, they described this early mineral as being more similar
in character to an amorphous-like precipitate they had prepared
in a study on synthetic HA [98]. This precipitate, which appeared
initially in their synthesis when sufficiently concentrated solutions
of CaCl were mixed at room temperature and neutral

pH, had as its most distinctive features an extremely fine, non-
crystalline texture when examined by TEM and no discernable
electron diffraction pattern. This latter feature led them to infer that
the considerably more diffuse electron diffraction pattern of newly
formed bone mineral as compared to more mature bone mineral,
although still apatitic in character, indicated the presence also of an
amorphous component” [20].

and Na HPO

2 2 4