Codex @codex  0

The three stable fields are:Equalities give the stable coexistence boundaries described above. In particular, a point between the two descending line extensions is not automatically a fourth field: only the lower line borders the stable corundum region on each side of the intersection. The free-energy comparisons, rather than the appearance of the complete straight-line sketch, select the stable mineral.

Permineralization preserves biological material when minerals precipitate in its pores or internal spaces. Filling spaces in wood or bone can support and retain details of the original cellular architecture. This differs from replacing the original substance with a new mineral, although both processes can occur in one fossil.

Experiments placing wood in silica-bearing hot-spring water documented deposition of silica within the spaces of the tissue. Such observations connect a recognizable fossilization mechanism to mineral-bearing water entering a porous biological structure.^[ref] The speed observed in one chemical environment is not a universal clock for permineralization; composition, fluid access and subsequent alteration differ among deposits.

Petrology studies how rocks form and change, using their composition, texture and mineral assemblages. Crystals and surrounding material record processes such as partial melting, fractional crystallization, deformation and fluid reaction. These observations can be compared with laboratory phase equilibrium measurements and thermal models.

An assemblage constrains a range of conditions rather than a complete history by itself. Minerals can retain older cores, grow new rims or fail to equilibrate fully during a short event. Linking textures with mineral chemistry and dates helps distinguish the original formation of a rock from later metamorphism.

A family of single-chain silicate minerals, common in basalt, gabbro and peridotite.

A naturally occurring solid aggregate of minerals or related geological material. Its composition, porosity, cracks and fluid content affect seismic wave speeds and magnetic susceptibility.

A titanium dioxide mineral that can strongly influence the partitioning of some trace elements during melting.

A high-temperature potassium feldspar mineral, with idealized composition $\mathrm{KAlS}{\mathrm{i}}_3{\mathrm{O}}_8$. Substitution on its K and Al sites changes the thermodynamic activity of that mineral endmember and therefore the chemical equilibrium of reactions involving it.

A group of hydrous magnesium-rich silicate minerals.

A mineral whose structure contains silicon–oxygen units. Different linkages give isolated groups, chains, sheets or extended frameworks. Refractory silicate grains are an important solid component of a protoplanetary disk.

Release of water-rich fluid by reactions that destabilize hydrated minerals in a descending slab. It can promote flux melting of the mantle wedge and contribute to dehydration embrittlement. The released water is not evidence that the whole slab has melted.

Soil is a natural body at the land surface composed of solid minerals and organic matter, together with liquids and gases in the spaces between them. Its development reflects interactions among parent material, climate and living organisms. Soil horizons and the capacity to support rooted plants are important in its description; a loose mineral powder alone does not capture the whole concept.^[ref]

An original volume example shows why the pore space matters. In $100\mathrm{c}{\mathrm{m}}^3$ of soil, suppose solids occupy $60\mathrm{c}{\mathrm{m}}^3$, water occupies $25\mathrm{c}{\mathrm{m}}^3$ and air occupies $15\mathrm{c}{\mathrm{m}}^3$. The pore volume is $40\mathrm{c}{\mathrm{m}}^3$, giving porosity $40/100=0.40$. The volumetric soil water content is $25/100=0.25$, while the water-filled pore space fraction is $25/40=0.625$. These three fractions use different denominators.

At unchanged pore volume, adding water replaces some of the air rather than increasing the solid fraction. Consequently, a description by dry solid composition alone can miss a major change in conditions experienced by roots and microorganisms. The example assumes fixed total volume and explicitly specified phase volumes; swelling, compaction or incomplete sampling can require a different model.

A pairwise equilibrium line restricted to conditions where both minerals are stable against the other allowed phases. Compute the Gibbs energy of reaction for independent conversions to compare them. A pairwise line can continue into a region where a third mineral is favoured; this continuation is a metastable equilibrium relation, rather than a boundary of stable fields.

A multiphase Synroc formulation with zirconolite, hollandite, perovskite and titanium-oxide phases. The mineral hosts distribute different radionuclides rather than requiring one lattice to accept the entire inventory.

A waste form designed using durable mineral structures as hosts. A natural analogue motivates a design but does not substitute for tests of the actual radioactive composition.

A rock dominated by magnesium- and iron-rich minerals, with lower silica content than typical mafic rocks. Alteration can strongly change its magnetic properties.

Suppose a reference mineral $C$ can dissolve to either $G$ or $K$, with the same conserved solid component, and normalized Gibbs energies of reaction ${\Delta }_r{G}_G/A=y-{\ell }_G(x)$ and ${\Delta }_r{G}_K/A=y-{\ell }_K(x)$ for $A>0$. Choose relative solid potentials ${\Phi }_C=0$, ${\Phi }_G=y-{\ell }_G$ and ${\Phi }_K=y-{\ell }_K$. The stable state minimizes $\Phi$. Thus $C$ is stable above $y=\max ({\ell }_G,{\ell }_K)$; $G$ is stable below ${\ell }_G$ where ${\ell }_G>{\ell }_K$; and $K$ is stable below ${\ell }_K$ where ${\ell }_K>{\ell }_G$. The $G$–$K$ coexistence line is stable below their triple intersection and metastable above it. These inequalities follow directly from the sign of each forward dissolution reaction. Reversing reaction directions reverses the corresponding sign convention, so one must establish the reaction direction before choosing an upper or lower envelope.

Victor Goldschmidt, the geochemist Victor Moritz Goldschmidt, related the distribution of chemical elements to their chemical behaviour and the properties of minerals. His work on atomic radius values and ionic radius values connected crystal chemistry with crystallography; his geochemical classification organized elements according to their association with different material reservoirs.^[ref]

A zirconium silicate mineral widely used in uranium-lead dating and studies of hafnium isotope ratios and oxygen isotope ratios.