Thermogravimetric analysis linked to mass spectrometry
(TGA-MS) shows changes in mass, and identifies gases evolved, when
a material is heated. Heating to 600oC enabled samples
of bone to be classified as having a high (cod clythrum, deer antler,
whale periotic fin bone) or a low (porpoise ear bone, whale tympanic
bulla, whale ear bone) proportion of organic material (Fig.
1). At higher temperatures, the mineral phase of the bone decomposed.
High temperature X-ray diffraction (HTXRD) showed that the main
solids produced by decomposition of mineral (in air or argon at
800-1000oC) were ß-tricalcium phosphate (TCP) and
hydroxyapatite (HAP), in deer antler, and CaO and HAP, in whale
tympanic bulla. In carbon dioxide, the decomposition was retarded,
indicating that the changes observed in air and argon were a result
of the loss of carbonate ions from the mineral. Fourier transform
infra-red (FTIR) spectroscopy, of bones heated to different temperatures,
showed that loss of carbon dioxide (as a result of decomposition
of carbonate ions) was accompanied by the appearance
of hydroxide ions. These results can be explained if the structure
of bone mineral is represented by
Ca10-xV(Ca)x[(PO4)6-x-y(HPO4)x(CO3)y][(OH)2-x-y(CO3)yV(OH)x
(1)
where V(Ca) and V(OH) correspond to vacancies
on the calcium and hydroxide sites, respectively, and 2-x-y = 0.4.
For deer antler with a total carbonate content of approximately
4% by mass, or y = 0.3, x must equal 1.3. These values of x and
y lead to a subscript of 0.4 for (OH)2-x-y (i.e. 20%
of the hydroxide content of stoichiometric HAP). Then formula
1 becomes:
Ca8.7(PO4)4.4(HPO4)1.3(CO3)0.3][(OH)0.4(CO3)0.3]
(2)
This composition corresponds to a Ca/P molar ratio of 1.53, which
is consistent with the observed phase decomposition on heating.
When CO2 is lost from the B-site on heating, a calcium-deficient
apatite will be produced that will decompose to HAP and TCP.
For whale tympanic bulla, with a carbonate content of approximately
8%, by mass, or y=0.65, x must equal 0.95, to give:
Ca9.05(PO4)4.4(HPO4)0.95(CO3)0.65][(OH)0.4(CO3)0.65]
(3)
This composition corresponds to a Ca/P molar ratio of 1.69, which
is consistent with the decomposition observed on heating in which
calcium oxide is produced. In decomposition of the structure represented
by formula 3, carbon dioxide is lost from the
B-site, producing a calcium-rich apatite, i.e. calcium oxide and
HAP remain. Table 1 shows the composition of the
various bones in general terms and, for comparison in Table
2, similar data obtained from human cancellous bone.
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Fig 1. TGA results in air.
Curves represent the highly mineralised porpoise ear bone, whale
tympanic bulla, whale ear bone and those with lower mineralisation,
whale periotic fin bone, deer antler and cod clythrum. Human osteoarthritic,
osteoporotic and control are included to show they lie with the
high organic types of bone. Masses were divided by the initial mass,
to give a relative mass, and are plotted against temperature
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