= Mineralized tissue
<Mineralized tissue> is biological <tissue> in which mineral forms a substantial structural component of an organic matrix. <Bone>, <dentine> and <enamel> are familiar examples, but they differ markedly in composition and architecture. In <bone>, mineral based on <hydroxyapatite> is associated with a <collagen>-rich <extracellular matrix>; <biological cells> form and maintain that matrix. Mineralization describes a material feature, not the absence of living activity.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 6.3, Bone Cells and Tissue, Figures 6.11–6.12: mineralized collagen matrix, bone cells and lamellar organization]{ref}
A <mineralized tissue> is therefore also a biological <composite material>. Mineral amount, organic matrix, water, porosity and orientation can each affect the response. The <Young's modulus> of an entire specimen cannot be inferred from mineral content alone: constituent geometry changes load sharing, just as it does in the <isostress rule of mixtures> and the <longitudinal rule of mixtures>. These ideal mixture models clarify the role of geometry without being exact models of a real <bone>.
Even the phrase “mineral fraction” needs a specified measurement. Consider a hypothetical dry two-component specimen with $6\,\mathrm{mg}$ of mineral of <density> $3.0\,\mathrm{mg/mm^3}$ and $4\,\mathrm{mg}$ of organic material of <density> $1.3\,\mathrm{mg/mm^3}$. Its mineral mass fraction is $0.60$, but its mineral <volume fraction> is
$$
\phi_m=\frac{6/3.0}{6/3.0+4/1.3}\approx0.394.
$$
These chosen numbers describe a bookkeeping example, not a measured tissue composition. Adding water or pores changes which total volume belongs in the denominator. A mechanical mixture calculation needs the appropriate <volume fractions>, not a mass percentage silently substituted for them.
An instructive primary structural study examined elephant ivory, a form of <dentine>, with complementary X-ray scattering, polarized-light <microscopy> and <scanning electron microscopy>. The investigators found that mineralized <collagen> orientations differed between the outer and inner parts of the tusk, including an outer wrapping orientation and a predominantly axial, plywood-like arrangement farther inside.<The three-dimensional arrangement of the mineralized collagen fibers in elephant ivory and its relation to mechanical and optical properties by M. Albéric and others>[Primary abstract, complementary methods and region-dependent fibre orientations]{ref} The result motivates combining methods: a bright pattern in polarized light, an electron-microscope image and a scattering pattern provide different evidence about the same organized <mineralized tissue>. Observing orientation is not by itself a direct measurement of its full mechanical <constitutive equation>.
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