Geol 202 The Common Ferromagnesian Minerals

What are the minerals forsterite and fayalite? How are they related to what we typically refer to as the mineral 'olivine'?

Olivine is a relatively simple mineral which has compositions that vary continuously in perfect solid solution between the end members forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Fosterite has Mg and Fayalite had Fe

What are exsolution lamellae in pyroxene? How do they form?

These intimate intergrowths of orthopyroxene (low-Ca pyroxene) and clinopyroxene (high-Ca pyroxene) are produced by exsolution, which is e"ectively an unmixing process that occurs in the solid-state, at low temperatures during cooling . The closey-spaced intergrowths of cpx and opx produced by exsolution are called exsolution lamellae. In general, the term 'lamellae' is used todescribe closely space and generally parallel intergrowths of two minerals. Slide 31 32

How can olivine, augite and hyperstene be distinguished from one-another on the basis of their appearance in a thin section.

they all have high interference colors augite has strong second order colors same with olivine

Describe the classification system for the pyroxene minerals. Be sure to include the triangular classification diagram (with end-members) and the pyroxene quadrilateral, as well as the ideas of high-Ca and low-Ca pyroxenes and their relationships to orthopyroxenes and clinopyroxenese. Be sure to also indicate what the common pyroxene names are (augite, pigeonite, etc.).

Pyroxenes are are inosilicate (chain silicates). Their compositions fall in the three end-member system enstatite, ferrosilite and wollastonite, as shown in the triangular diagram. Slide 19-20 Within the pyroxene quadrilateral, there are orthorhombic pyroxenes (orthopyroxenes) which have low Ca and include the common Mg-rich compositions of enstatite, bronzite and hyperstene. The pyroxene quadrilateral also includes the monoclinic pyroxenes (clinopyroxenes) which have high Ca and include the common minerals diopside, augite and pigeonite.

What is illustrated by these figures? How are they related?

Slide 22 The top figure here illustrates the formation of the single chains by linking SiO4 tetrahedra by two co-planar oxygens. The lower diagram shows the end-view of the beam-shaped structures created by the single chains. Notice that the beam-shaped structures are stacked top-to-top and bottom-to-bottom, and that they are held together by various cations (Mg,Fe,Ca,Na) which occupy octahedral sites (labeled M1 and M2 in the figure) between the beams. Notice also that the tetrahedral sites are occupied by Si and Al. Slide 25 These figures illustrate the prismatic cleavage of pyroxenes

Be prepared to interpret the evolution of an pyroxene composition for super-solvus to sub-solvus conditions, using the simplified pyroxene solvus diagram presented in class

Slide 33 34 35

Pyroxene and olivine minerals are all silicates, so all are constructed from the basic [SiO4]4- building block. Notice however that that the chemical formula for olivine includes an SiO4 unit, but the silicate unit in the chemical formula for the pyroxenes includes either an SiO3 or Si2O6 unit. Why is this the case? Why do the silicate units in the chemical formulae for olivine and pyroxene minerals differ in this way?

Slide 41 and 42

Be prepared to use the olivine phase diagram (below) to determine the melting point of any olivine composition and the compositions of olivine crystals and melts that are in equilibrium with one another at any temperature on the diagram

Slide 7

Briefly describe and illustrate the nature of the amphibole crystal structure. Based on this structure, explain why the amphiboles are such a chemically complex mineral group.

The amphiboles are double-chain silicates. They are constructed from single chains of linked tetrahedras that are cross-linked to produce the double-chain structure. This type tetrahedral linking produces a Si ratio to O of 8:22 The crystal structures of the amphiboles are similar to those of the pyroxenes but with double-chain tetrahedral site "beams" that are stacked top-to-top and bottom-to-bottom and with octahedral sites between them. There are more different sizes of octahedral sites, and some larger octahedral sites in the amphiboles than in the pyroxenes, so lots of different cations can fit into amphibole crystal structures, including significant quantities of K, which is quite a large atom. The structural complexity of amphiboles produces chemical complexity and makes amphibole classification complex and difficult Slides 41 and 42

What is the nature of the data in this table? Briefly summarize and describe all aspects of these data as they were discussed in class.

The chemical compositions of six pyroxenes are shown here. The top columns of data are the compositions expressed in oxide weight percent. The bottom columns of data show the same compositions re-calculated on an atomic basis to show the relative abundances of the cations assuming that the unit cell has 6 oxygens. In this form, the data express the relative proportions of the cations that are in the chemical formula; that is, two atoms of Si and two atoms of Mg+Fe +Ca and four total cations per four oxygens. The WO, FE and EN notation at the bottom of the table indicates the relative atomic proportions of the wollastonite, ferrosilite and enstatite end-members.