Mineral assembly involves the formation of ionic or covalent bonds between cations and anions to create a repeatable crystalline framework. This framework can have isotropic or anisotropic properties depending on consistent or varying structures in different directions. Mineral growth occurs through mechanisms like nucleation and Ostwald ripening as ions come together from melts or solutions to form larger crystals in equilibrium. Polymorphs exist when the same chemical formula produces different mineral structures under varying conditions of formation.
Intze Overhead Water Tank Design by Working Stress - IS Method.pdf
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Lecture 5 - Crystallization.ppt
1. Mineral assembly
โข Most minerals will deal with ionic bonds
between cations and anions (or anionic
subunits which are themselves mostly
covalent but do not dissociate)
โข Assembly of minerals can be viewed as
the assembly of individual ions/subunits
into a repeatable framework
โข This repeatable framework is a crystal or
crystalline material
2. Mineral Assembly
โข Isotropic โ same properties in every
direction
โข Anisotropic- different properties in different
directions ๏ most minerals are this type
โข Assembly of ions from melts, water, or
replacement reactions which form bonds
โข The matrices the ions are in always contain
many different ions โ different conditions of
formation for the same mineral creates
differencesโฆ
3. Polymorphs
โข Two minerals with the same chemical formula but
different chemical structures
โข What can cause these transitions??
โขsphalerite-wurtzite
โขpyrite-marcasite
โขcalcite-aragonite
โขQuartz forms (10)
โขdiamond-graphite
4. Complexes ๏ Minerals
โข Metals in solution are coordinated with ligands
(Such as H2O, Cl-, etc.)
โข Formation of a sulfide mineral requires direct
bonding between metals and sulfide
โ requires displacement of these ligands and
deprotonation of the sulfide
โข Cluster development is the result of these
requirements
5.
6.
7.
8. Mineral growth
โข Ions come together in a crystal โ charge is
balanced across the whole
โข How do we get large crystals??
โ Different mechanisms for the growth of
particular minerals
โ All a balance of kinetics (how fast) and
thermodynamics (most stable)
9. Nucleation
โข Aggregation of molecules builds larger and
larger molecules โ becomes a nucleus at
some point
โข Nucleus โ size of this is either big enough
to continue growth or will re-dissolve
(Critical Size)
โข Overall rate of nucleus formation vs.
crystal growth determines crystal
size/distribution
10. Crystal Shapes
โข Shape is determined by atomic
arrangements
โข Some directions grow faster than others
โข Morphology can be distinct for the
conditions and speed of mineral
nucleation/growth (and growth along specific
axes)
11. Ostwald Ripening
Larger crystals are more stable than smaller crystals
โ 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 โfeedingโ the
larger ones through a series of dissolution and
precipitation reactions
12. Figure 3-17. โOstwald ripeningโ in a monomineralic material. Grain boundaries with significant
negative curvature (concave inward) migrate toward their center of curvature, thus eliminating
smaller grains and establishing a uniformly coarse-grained equilibrium texture with 120o grain
intersections (polygonal mosaic). ยฉ John Winter and Prentice Hall
13. Small crystalsโฆ
โข In the absence of ripening, get a lot of very
small crystals forming and no larger
crystals.
โข This results in a more massive
arrangement
โข Microcrystalline examples (Chert)
โข Massive deposits (common in ore
deposits)
14. Topotactic Alignment
โขAlignment of smaller grains in space โ due to magnetic
attraction, alignment due to biological activity (some microbes
make a compass with certain minerals), or chemical/
structural alignment โ aka oriented attachment
15. Igneous Textures
Figure 3-1. Idealized rates of
crystal nucleation and growth as
a function of temperature below
the melting point. Slow cooling
results in only minor
undercooling (Ta), so that rapid
growth and slow nucleation
produce fewer coarse-grained
crystals. Rapid cooling permits
more undercooling (Tb), so that
slower growth and rapid
nucleation produce many fine-
grained crystals. Very rapid
cooling involves little if any
nucleation or growth (Tc)
producing a glass.