INTRODUCTION TO MINERALOGY 11

Transkript

1 INTRODUCTION TO MINERALOGY 11 OXIDES The bonds to oxygen are more stable if they have a higher covalent character. Such metals form more stable simple oxides. Al and Fe oxides are therefore more common than Mg-oxide, while pure Ca, Na and K oxides are quite uncommon. The complex oxides, where the difference in the character of bonds is large, are grouped after the strongest bond as silicates, phosphates, sulphates, carbonates etc.

2 INTRODUCTION TO MINERALOGY 11 EUTAXY (the closest packing) Two simplest eutaxies are the hexagonal, and the cubic close packing of spheres (hcp, and ccp). They have the same type of closepacked layers, but the stacking of layers are ABAB, and ABCABC, respectively. Note the octahedral and the tetrahedral coordination holes, and that there are four equivalent layer directions i the ccp (parallel to octahedral faces) giving it the cubic symmetry.

3 INTRODUCTION TO MINERALOGY 11 Hematite-corundum group Hematite (Fe 2 O 3 ) Corundum (Al 2 O 3 ) Ilmenite (FeTiO 3 ) Red streak and metalic lustre (opaque in microscope) of hematite. Weak magnetism of hematite and ilmenite. Structure based on a near-hcp of oxygen atoms. Cations fill 2/3 of the available octahedral holes.

4 INTRODUCTION TO MINERALOGY 11 Periclase group Periclase (MgO) Wustite (FeO) Important in the lower mantle, rare in lithosphere. Structure based on the ccp of oxygen atoms. Cations fill all of the available octahedral voids. This is the NaCl structure type.

5 INTRODUCTION TO MINERALOGY 11 Spinel group Large group of complex oxides with cations in two valence states. Examples: Spinel (Mg II Al 2 IV O 4 ) Magnetite (Fe II Fe 2 III O 4 ) Ulvöspinel (Fe 2 II Ti IV O 4 ) Srtucture based on the near-ccp of oxygen atoms. Cations are distributed in both octahedral and tetrahedral voids. Common in rocks formed in the deep lithosphere and upper mantle. Accessory minerals in ultrabasic rocks. Large magnetite masses in precambrian metamorphic iron provinces. There are double so many atoms in octahedral as in tetrahedral coordinations.

6 INTRODUCTION TO MINERALOGY 11 Hydrogen in minerals HYDROXIDES WATER Most important hydroxide minerals are hydroxides of Fe, Al, and Mn. The first two groups make economically important rocks limonite and bauxite. They mostly contain polymorphs of FeOOH, and ALOOH. The structure of isomorphs goethite and diaspore is shown. Ice. The molecules of water are held together by hydrogen bonds. Water in other crystals is also attached by such bonds to the anionic framework of oxygen atoms.

7 INTRODUCTION TO MINERALOGY 11 XO 2 structures Rutile most important representatives Quartz Typical for octahedrally coordinated cations. Tetragonal symmetry. The high-pressure form of SiO2 (stishovite) has the same structure. Unique among common minerals. Trigonal trapezohedral symmetry. Enantiomorphic. One of dominating minerals in the upper lithosphere. Important source of Si, and important piesoelectric material.

8 INTRODUCTION TO MINERALOGY 11 SiO 2 phase system Beta-quartz is mostly formed in igneous and high-grade metamorphic rocks. Transforms readily to alphaquartz on cooling. Tridimite and cristobalite are not so rare in volcanic rocks. In the mantle the high-pressure forms are formed. On the surface found only in meteorite craters and mantle xenolites. In sediments also microcrystalline alpha-quartz varieties (calcedone) and amorphous opale are found.

9 INTRODUCTION TO MINERALOGY 12 SILICATES CLASSIFICATION Nesosilicates Inosilicates Phyllosilicates Tectosilicates

10 INTRODUCTION TO MINERALOGY 12 The structure and properties of a silicate depend as much on the role of other cations in the structure. Usually the srtucture is selective, and the different cations prefer different silicates. A rare example is amphibole which accomodates practically all types of cation coordinations. Because of that, when amphibole forms it takes in its composition almost everything from the environment and is therefore called the mineralogical garbage can.

11 FELDSPATER KAlSi 3 O 8 NaAlSi 3 O 8 CaAl 2 Si 2 O 8 Opbyger hovedparten af Jordens skorpe. To (petrografiske) hovedgrupper: -alkaliske feldspater -plagioklaser Sammen med kvarts er felsiske (lysfarvede) mineraler i hovedparten af magmatiske bjergarter (undtagen de ekstrem Si-arme varieteter). Det samme gælder for siliciske metamorfe bjergarter. Ustabile i det sedimentære miljø. Findes i grauvaker og arkozer, men bliver erstattet af lermineraler i mere modne klastiter. Stabile til lidt over 1000 o C og 4 GPa og derfor begrænset til 30 km dybde.

12 INTRODUCTION TO MINERALOGY 12 Feldspars have high- and low-forms which depend on the ordering grade of Al and Si in the structure. They represent different polymorphs with different properties and can be used to classify rocks as igneous or volcanic, and estimate the closure temperatures. Sanidine Microcline

13 KRYSTALLOGRAFI OG TEKSTURER Hovedparten er trikline. Monokline varieteter kan praktisk findes kun imellem K-rige feldspater. Findes meget oft i tvillinger og har flere karakteristiske tvillingelover. Typiske er polysyntetiske tvillingelameller efter albit- eller periklinlov. Tætte sammenvoksninger af to feldspater med to forskellige sammensætning er ligeledes meget typiske. Tvillingelameller efter To systemer af tvillingelameller Zonaropbygning af

14 PERTHIT OG TERNARE FELDSPATER HREM billede af en perthit. Plagioklas lameller i K- feldspat. Bemærk albit-tvillingelameller i plagioklaslamellerne. Fasediagram for de alkaliske feldspater. Bemærk afblandingsregion ved under 650 o C med afblanding i en K-rig monoklin feldspat plus den trikline albit. Højtemperatur monoklin feldspat som ligger mellem sanidin og monalbit kaldes også anorthoclas. Hvide plagioklaslameller i den rødlige K- feldspat.

15 PLAGIOKLAS GENNEMVOKSNINGER Plagioklas-fasesystem. Bemærk liquidussolidus som beskriver den normale krystalliseringsekvens i magma. Der er en næsten fuldstændig blandbarhed ved temperaturer over 800 o C med strukturændring fra en triklin-c plagioklas til en triklin-i Ca-rig plagioklas. Ved lavere temperaturer er der tre afblandingsregioner mellem albit moduleret pg2 moduleret pg1 anorthit som ordnede varianter. En labradoriserende plagioklas og en månesten Lameller af An 2 i An 15 i en lavtemperatur plagioklas fra peristerit regionen.

16 INTRODUCTION TO MINERALOGY 12 Feldspathoids Similar to feldspars in chemical composition, but contain more alkalis and less SiO 2. They are typical for highly alkaline rocks. Some feldspatoids contain very large cavities in the alumo-silicate framework, and can easily exchange cations (zeolites), or contain additional anions (like SO 4, Cl, and S in sodalites). In alkaline rocks nepheline is tipically found instead of albite. Note that it has two different sites for alkaline cations. Nepheline (KNa 3 [AlSiO 4 ] 4 ) has hexagonal symmetry