Codex @codex  0

A colloidal assembly of caseins associated with calcium phosphate in milk; changes in pH and enzyme action can destabilize it.

Hydroxyapatite is a crystalline calcium phosphate whose ideal composition is $\mathrm{C}{\mathrm{a}}_{10}(\mathrm{P}{\mathrm{O}}_4{)}_6(\mathrm{OH}{)}_2$, equivalently $\mathrm{C}{\mathrm{a}}_5(\mathrm{P}{\mathrm{O}}_4{)}_3\mathrm{OH}$. The formula describes an extended ionic solid, not a discrete hydroxyapatite molecule. Its ideal calcium-to-phosphorus molar ratio is $10/6=5/3$. NIST uses hydroxyapatite reference powder to evaluate physical and chemical measurements of apatites.^[ref]

The ideal composition is electrically neutral: ten calcium ions contribute charge $+20$, six phosphate ions contribute $-18$ and two hydroxide ions contribute $-2$. Changing a subscript changes this composition; it is not the same operation as multiplying the whole chemical formula by a coefficient.

An original composition calculation uses approximate molar masses $40.078$ for calcium, $30.974$ for phosphorus, $15.999$ for oxygen and $1.008$ for hydrogen, in grams per mole. The full formula has molar mass about $1004.62\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$, with ideal calcium mass fraction about $39.89\%$ and phosphorus mass fraction about $18.50\%$. Their mass-fraction ratio is about $2.157$, whereas their mole ratio is $1.667$. An elemental mass ratio cannot be compared directly with the stoichiometric mole ratio.

Identity also needs structural evidence. NIST characterized its reference powder by elemental analysis and powder X-ray diffraction, comparing diffraction positions with the hydroxyapatite pattern. Its certificate distinguishes certified composition values from supplementary diffraction information.^[ref] A measured Ca/P ratio near $5/3$ alone does not establish that every part of a specimen is a single hydroxyapatite phase: a mixture can reproduce an overall elemental ratio. Composition, phase identification and the properties of a biological composite material answer related but different questions.

Cow's milk contains water, dispersed fat, casein, soluble milk proteins, lactose and mineral constituents, with smaller amounts of other substances. These constituents occupy different physical states, which makes their separation possible: the fat is present as an emulsion, much of the casein as colloidal casein micelles, and the lactose and many salts in chemical solution. Vitamins, enzymes and small nitrogenous constituents accompany these main fractions. The proportions vary between samples and are not fixed by the name cow's milk.^[ref]

First let fresh unhomogenized milk stand so that the less dense fat globules rise as cream; skimming gives a fat-enriched layer and skimmed milk beneath. This separates fractions rather than pure chemical substances: cream still contains water and other constituents. Churning cream coalesces the fat globules into butter; extraction of a separately prepared dried portion with a suitable organic solvent provides another way to isolate fat. A modern centrifugal separator accelerates the same density-based separation, but it is not needed to explain an 1864 bench separation.

Next add dilute acetic acid carefully to skimmed milk. Lowering its pH toward the isoelectric point of casein, about 4.6, produces a curd that can be collected by filtration and washed. Excess acid, retained fat, and shifts of calcium phosphate between curd and liquid limit purity. Rennet offers another route to a curd, but its enzyme-catalysed action and the mineral content of the resulting curd differ from simple acid precipitation; the two should not be treated as identical mechanisms.^[ref]

The liquid after removal of the curd is whey. Heat this fraction under suitable acidity to induce protein denaturation and aggregate much of its residual whey protein; collect the coagulum by filtration. Older descriptions commonly grouped this fraction under “albumin”, but modern milk chemistry distinguishes several proteins. Heating alone does not make every minor constituent fall into a single pure fraction.^[ref]

Concentrate the deproteinized whey and cool it to crystallize lactose. Filter and wash the crystals, then purify them by recrystallization; dissolved salts and residual organic matter remain substantially in the mother liquor. This provides a separation from most protein, not an automatic complete separation from all minerals.^[ref]

Finally use separate measured portions for water and mineral determinations. Controlled drying gives an operational total solids measurement; subtraction from the initial mass estimates the water lost. Ignition of a dried portion destroys most organic matter and leaves ash for mineral analysis. Ash is a transformed residue: it does not preserve all original salts unchanged, and some constituents can volatilize. Calcium, phosphorus, sodium, potassium and chloride are among the important mineral constituents. Skimming, curd formation, heat coagulation, lactose crystallization, drying and ashing separate or determine the principal fractions; none alone constitutes a complete pure-component analysis.

Gallstones contain chiefly cholesterol and/or bile pigment, with variable calcium salts and an organic matrix. They are mixtures with different predominant components, not one compound with a universal formula. The period word cholesterin denotes cholesterol.

Cholesterol-rich stones contain crystalline cholesterol, often with a smaller pigmented nucleus and layers of other material. Mixed stones combine cholesterol, pigment, calcium salts and an organic matrix including mucin. Pigment stones contain much less cholesterol; a major constituent is calcium bilirubinate, related to the bile pigment bilirubin. Minor inorganic constituents can include calcium carbonate and calcium phosphate. Not every visibly dark stone has the same composition, and no fixed percentage follows from the question.^[ref]

The useful modern subdivision of pigment stones is into black and brown types. Black stones commonly contain polymerised pigment with calcium salts; brown stones include calcium bilirubinate and calcium salts of fatty acids, particularly in infected stagnant duct bile. These refinements explain why neither “all stones are cholesterol” nor “all stones are lime salts” is adequate.^[ref]

Chemical examination should distinguish the fractions rather than infer them merely from the external colour. Cholesterol can be extracted with a suitable organic solvent and recognised in recrystallised material; microscopy can show characteristic plate-like crystals. A mineral residue after removal or destruction of organic matter indicates an inorganic fraction, but residue alone cannot identify a particular salt. Layering, facets and colour describe growth and packing and are not chemical analyses. In particular, calcium bilirubinate is not interchangeable with calcium carbonate, and cholesterol is not a bile salt.

taphonomy studies the transformation of biological remains from death or production into the material ultimately recovered and interpreted. It explains how a living organism or community becomes an incomplete geological sample. For shed structures or trace fossils, production of the evidence need not involve the death of the producer. Its relevance therefore extends beyond asking whether a skeleton was buried intact.

Three primary process classes are decay; physical disarticulation and transport; and burial alteration, including diagenetic mineralization. The boundaries are operational and the mechanisms interact: post-mortem decay can continue after burial, and previously buried remains can be reworked and transported again. biostratinomy groups events before final burial, whereas diagenesis groups subsequent physical and chemical changes. Those are stages or settings, rather than three mutually exclusive mechanisms.

First, post-mortem decay transforms tissues by autolysis, microbial decomposition and associated chemical reactions. Soft tissues, connective tissues and different phylogenetic characters disappear at different rates. The important result is not simply that hard parts survive longer: a partly decayed organism can acquire a misleading anatomical combination. Experiments on amphioxus and lamprey larvae followed recognizable characters through decomposition. Derived diagnostic features tended to disappear before some more general chordate features, producing a directional bias. Coding every unobserved diagnostic character as ancestrally absent can push a genuinely derived fossil toward an earlier position on a phylogenetic tree, the effect called stemward slippage.^[ref]

Experiments on adult hagfish and lampreys extended the comparison beyond larvae. They documented transformations as well as disappearance, so a residual stain or collapsed structure should not automatically be identified as an intact organ. Replicate specimens sampled through time are needed because destructive observations cannot follow every internal character in the same individual. For a new experimental test, temperature, water chemistry, microbial conditions and starting condition should be recorded and, where tested, varied independently. A laboratory sequence supplies constraints on interpretations; it is not a universal clock assigning an exact post-mortem duration to every fossil.^[ref]

Second, disarticulation, fragmentation and taphonomic transport alter which elements occur together and where they accumulate. Loss of ligaments releases bones, while currents or scavengers can move them. Shape, density, size, articulation and orientation influence transportability, so a concentration of robust elements need not represent the most abundant living species or the original place of death. Flow experiments and comparison of element representation, as developed in Michael R. Voorhies's vertebrate assemblage study, provide a basis for testing such alternatives. A useful comparison measures elements under specified flow and sediment conditions, with articulated versus disarticulated material distinguished. hydraulic sorting cannot be transferred unchanged between all animals and environments.^[ref]

The consequences include mixing habitats and ages. An apparently coherent bone bed may combine local and transported animals; reworking of fossils can place older remains in younger sediment; time averaging can combine successive communities into one assemblage. None implies that every fossil assemblage is hopelessly mixed. Articulation, abrasion, orientation, size distributions, sedimentary context and independent age information can instead delimit the plausible history. Abrasion alone is an imperfect transport-distance meter because material properties and the transport medium also matter.

Third, burial changes both preservation and geometry. Rapid isolation can reduce scavenging, but burial does not halt all decomposition. sedimentary compaction flattens and fractures remains; dissolution removes mineralized tissues; precipitation can infill pores or replicate tissues; fossil recrystallization can erase original microscopic detail. A fossil can therefore retain an excellent outline but lack its original chemistry, or preserve histology while its overall shape has been distorted. Mineralized replacement is not the only route to fossilization, and surviving original hard parts need not have undergone complete replacement.

Experiments by Derek E. G. Briggs and Amanda J. Kear illustrate constructive as well as destructive diagenesis. Decaying shrimps developed diagenetic mineralization by calcium phosphate preserving muscle detail, particularly in oxygen-restricted systems, with the animal itself supplying phosphate. diagenetic mineralization began on a timescale of weeks. This demonstrates a mechanism by which decay-associated chemistry can stabilize fine structures before they disappear. It does not show that low oxygen guarantees preservation: chemical availability, acidity, tissue composition and the relative rates of destruction and precipitation still control the outcome.^[ref]

These mechanisms are indispensable for reconstructing the history of life. A fossil's first occurrence is a minimum observation of a lineage, not automatically its evolutionary origin. A sudden appearance of soft-bodied organisms can reflect a new window of exceptional preservation, while disappearance of a phylogenetic character can reflect its destruction rather than evolutionary loss. Relative fossil abundance reflects both biological abundance and the probabilities of production, preservation, exposure and recovery. Comparisons of diversity through time must therefore consider changes in environments, preservational pathways and sampling. The fossil record is filtered evidence whose reliability can be tested; taphonomy identifies which biological conclusions that evidence can support.

Preservation or replication by calcium phosphate minerals, sometimes retaining soft-tissue detail when precipitation occurs before its destruction.