Mineral assembly

2. Mineral assembly • Most minerals will deal with ionic bonds between cations and anions (or anionic subunits which are themselves mostly covalent but do not dissociate) • Assembly of minerals can be viewed as the assembly of individual ions/subunits into a repeatable framework • This repeatable framework is a crystal or crystalline material

3. Mineral Assembly • Isotropic – same properties in every direction • Anisotropic- different properties in different directions  most minerals are this type • Assembly of ions from melts, water, or replacement reactions which form bonds • The matrices the ions are in always contain many different ions – different conditions of formation for the same mineral creates differences…

4. Polymorphs • Two minerals with the same chemical formula but different chemical structures • What can cause these transitions?? • sphalerite-wurtzite • pyrite-marcasite • calcite-aragonite • Quartz forms (10) • diamond-graphite

5. Complexes  Minerals • Metals in solution are coordinated with ligands (Such as H2O, Cl-, etc.) • Formation of a sulfide mineral requires direct bonding between metals and sulfide • requires displacement of these ligands and deprotonation of the sulfide • Cluster development is the result of these requirements

9. Mineral growth • Ions come together in a crystal – charge is balanced across the whole • How do we get large crystals?? • Different mechanisms for the growth of particular minerals • All a balance of kinetics (how fast) and thermodynamics (most stable) • Nucleation of small particles is generally a kinetically favored process – form fast, but also redissolve…

10. Ostwald Ripening Larger crystals are more stable than smaller crystals – the energy of a system will naturally trend towards the formation of larger crystals at the expense of smaller ones In a sense, the smaller crystals are ‘feeding’ the larger ones through a series of dissolution and precipitation reactions

11. Small crystals… • In the absence of ripening, get a lot of very small crystals forming and no larger crystals. • This results in a more massive arrangement • Microcrystalline examples (Chert) • Massive deposits (common in ore deposits)

12. Topotactic Alignment • Alignment of smaller grains in space – due to magnetic attraction, alignment due to biological activity (some microbes make a compass with certain minerals), or chemical/ structural alignment – aka oriented attachment

13. Imperfections • Further effects on minerals associated with formation: • Zonation – form concentric rings or shells in which the composition or T-P conditions change during crystallization • Twinning – same kind of mineral with different alignments – commonly start from one point or line and grow out in different directions

14. Zoning • Can be minor or major differences reflected in zones containing different phases, colors, or trace element compositions

15. Twinning Albite twinning a.k.a. polysynthetic twinning – occasionally visible in hand specimen -characteristic of all feldspars (Albite is a kind of feldspar, this characteristic happens to be named after it) Usually visible in thin section

16. Albite twinning • This type of twinning is governed by a ‘twin law’, stating that the twins form parallel to each other, aligned along an optic axis • This alignment along an optic axis results in the twins being a measure of composition – how different types of twinned feldspars interact with light

19. Crystal Chem  Crystallography • Chemistry behind minerals and how they are assembled • Bonding properties and ideas governing how atoms go together • Mineral assembly – precipitation/ crystallization and defects from that • Now we will start to look at how to look at, and work with, the repeatable structures which define minerals. • This describes how the mineral is assembled on a larger scale