Contiene anillos simples y dobles de seis tetraedros que se disponen perpendicularmente a un eje senario o ternario. Este grupo de minerales contiene cavidades de gran tamao que pueden describirse como octaedros o cuboctaedros truncados.
Presentan estructuras que pueden considerarse como lminas formadas por anillos de cinco tetraedros con uno anexado. Open navigation menu.
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Tectosilicatos Grupo de la Slice SiO2 La diferencia entre los minerales est en su estructura cristalina, existiendo tres minerales importantes: Aumento de Ta Aumento de Ta Estos tres minerales son polimorfos reconstructivos. Hatch y Ghose, Cristobalita de baja a : Tetragonal. Tectosilicatos Variedades criptocristalinas de Slice Calcedonia : gata con bandas paralelas a los bordes de colores vistosos.
Estos tetraedros presentan un ordenamiento en el cual los tetraedos varan en su posicin hacia arriba y hacia abajo Formas monoclnicas y triclnicas Angel et al. Lminas de seis tetraedros. Documents Similar To Silicatos - Tectosilicatos. Miguel Cav. Juan Crisostomo Ramos. Ruth Andromeda Lazarte Montesinos.
Jared Cruz. Luis Alonso Ocsa. Julio C. Guevara Diestra. Yuel Cobos Zamora. Pablo Andres Molina. Dayana Solange Jaramillo. Three membered rings, Si 3 O 9 -6 , four membered rings, Si 4 O 12 -8 , and five membered rings Si 5 O 15 are also possible. If two of the oxygens are shared in a way to make long single chains of linked SiO 4 tetrahedra, we get the single chain silicates or inosilicates.
In this case the basic structural unit is Si 2 O 6 -4 or SiO 3 If two chains are linked together so that each tetrahedral group shares 3 of its oxygens, we can from double chains, with the basic structural group being Si 4 O 11 The amphibole group of minerals are double chain silicates, for example the tremolite - ferroactinolite series - Ca 2 Mg,Fe 5 Si 8 O 22 OH 2. If 3 of the oxygens from each tetrahedral group are shared such that an infinite sheet of SiO 4 tetrahedra are shared we get the basis for the phyllosilicates or sheet silicates.
In this case the basic structural group is Si 2 O 5 The micas, clay minerals, chlorite, talc, and serpentine minerals are all based on this structure. Note that in this structure, Al is substituting for Si in one of the tetrahedral groups. Tectosilicates Framework Silicates. If all of the corner oxygens are shared with another SiO 4 tetrahedron, then a framework structure develops. The basic structural group then becomes SiO 2. The minerals quartz, cristobalite, and tridymite all are based on this structure.
Thus, the feldspar and feldspathoid minerals are also based on the tectosilicate framework. Based on these basic structural units, we can construct a general structural chemical formula for the silicates. But one substitution in particular tends to mess things up a bit. Thus, it could either fit in 6-fold coordination with oxygen or 4-fold coordination with oxygen. If such a substitution takes place, it creates a charge imbalance that must be made up elsewhere in the silicate structure.
The other common elements in the Earth's crust that enter the silicates do so in other types of coordination. O is oxygen, and W is a hyrdoxyl OH -1 site into which can substitute large anions like F -1 or Cl The subscripts m, n, and r depend on the ratio of p to q and are chosen to maintain charge balance.
This is summarized in the table shown here. In this table note that there is very little substitution that takes place between ions that enter the X, Y, and Z sites. Nesosilicates Island Silicates. We now turn our discussion to a systematic look at the most common rock forming minerals, starting with the common nesosilicates. Among these are the olivines, garnets, Al 2 SiO 5 minerals, staurolite, and sphene the latter two will be discussed in the last lecture on accessory minerals.
As discussed above, the nesosilicates or island silicates are based on the isolated SiO 4 -4 tetrahedral groups. There is limited substitution of the following end members:.
The phase diagram for the common end members of the olivine solid solution series shows that pure forsterite melts at o C and pure fayalite melts at o C. Occurrence Pure forsterite is limited to metamorphosed Mg-rich limestones and dolomitic metamorphic rocks. Fo 60 - 90 is found in basic igneous rocks likes basalts and gabbros, and sometimes in andesites, where it occurs with plagioclase and pyroxene. Fa - 40 is found in Fe-rich siliceous igneous rocks like rhyolites and granites. Mg-rich olivines rarely occur in quartz bearing rocks and quartz rarely occurs with Mg-rich olivine because the reaction shown below runs to the right for most pressures and temperatures.
Garnets are isometric minerals and thus isotropic in thin section, although sometimes they are seen to be weakly birefringent slightly anisotropic. They are also nesosilicates, and therefore based on the SiO 4 structural unit.
Garnets with no Ca in the A site and Al in the B site are called the pyralspite series. Occurrence The garnets occur mostly in metamorphic rocks where they are often seen to form euhedral well-formed crystals.
The Mg-rich garnet, pyrope, is found in metamorphic rocks formed at high pressure and in eclogites basalts metamorphosed at high pressure and peridotites ultrabasic rocks containing olivine, Opx, Cpx, and garnet. The Fe-rich garnet, almandine, is the most common garnet and is found in metamorphic aluminous schists. The Mn-rich variety, spessartine, is limited to Mn-rich metamorphic rocks like meta-cherts.
The Al 2 SiO 5 minerals are common in aluminous metamorphic rocks meta-shales and meta-mudstones and sometimes found in aluminous igneous rocks.
In metamorphic rocks the Al 2 SiO 5 polymorphs provide rather general estimates of the pressure and temperature of metamorphism, with Kyanite indicating relatively high pressure, andalusite indicating low temperature and pressure, and sillimanite indicating high temperature.
Better estimates of pressure and temperature are provided if two of the minerals are present in the same rock. Maximum birefringence is generally seen to be between 2 o yellow to 2 o red. Sillimanite is biaxial positive with a 2V of 21 - 31 o. Staurolite is a common mineral in medium grade metamorphic rocks, usually metamorphosed shales. In thin section Staurolite is commonly seen to contain tiny inclusions of other minerals, usually quartz.
There are very few minerals which can be confused with Staurolite. Zircon ZrSiO 4. Zircon is a common accessory mineral in nearly all kinds of rocks, particularly the more siliceous igneous rocks, like granites, granodiorites, and syenites. Still, it is not often found in thin section because it is so hard that it gets plucked out during the grinding of the section.
Zircon usually contains high amounts of radioactive elements like U and Th. Thus, when it is found as inclusions in minerals like biotite, it produces pleochroic haloes in the biotite as seen in thin section. Esto se ha visto conveniente para la comodidad del lector. Libro PDF Rating: 5. Etiquetas : Mineralogia y Cristalografia. Tweet Compartir Compartir Compartir Compartir. About Sweetheme Number of Entries :
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