Download Min_HW04_Triangular_Plots_16

Homework #4 – Mineralogy Geos 250 Name:________________________
Calculating Molecular or Atomic Proportions from Mineral Formulae or Weight %
Chemical Analyses and their Representation on Triangular Diagrams (projections)
Minerals participate in reactions: crystallization, precipitation, partial melting, recrystallization etc.
How they react with each other, with magmas or with fluids depends on specific components and
how they behave under changing environmental conditions (Pressure, Temperature, Oxidation
state/fugacity, Water availability, Chemical surroundings etc.). For example hotter magmas tend to
crystallize more Mg rich olivine or pyroxene while cooler ones form more Fe rich versions of the
same mineral. We have come to recognize some mineral settings by their composition e.g. low
temperature metamorphism favours Albite plagioclase while high temperature metamorphism
favours Anorthite. Comparing mineral analyses in a related suite of rock samples also tells us which
processes were active when they crystallized. Mineral formulae are already expressed in atomic or
molecular proportions so their conversion is relatively easy. Weight % analyses requires converting
component weights to atom counts by dividing by formula weights (grams per mole). We also have
to rearrange atoms by which part of the structure or formula they are found in. To this end we
combine minor contents of Zn or Cr with Mg or Fe that are the major elements in those octahedral
sites. Often Al must be partitioned between octahedral and tetrahedral sites so that formulae charge
balance as well as make structural sense.
1
2
3
Pyroxenes, while all single chain silicates have 2 or 3 different structures. Orthorhombic Pyroxenes
between Pure Enstatite MgSiO3 and pure Ortho-ferrosilite FeSiO3 lie near the bottom join of the
triangle. In real rocks, these can contain up to a few mol% of CaSiO3 . Ortho-ferrosilite is restricted
to Fe rich metamorphosed iron formations and furnace slags while Enstatite to Hypershene exist in
Peridotites, Gabbros and Basalts and high grade metamorphic rocks. Once there is more than a few
mole % of the Wollastonite molecule, Calcium is too big and forms a different Monoclinic Pyroxene
called Augite or Diopside for the Mg rich varieties and Hedenbergite for the Fe rich varieties.
Pyroxenes intermediate to these 2 types are found in andesites and dacites. For alkaline igneous
systems rich in Na, sodic ferrohedenbergite and acmite or aegerine form. Once Ca is greater than
Mg+Fe a high Ca monoclinic pyroxene form called Salite. Beyond this, in Ca rich calc-silicate
metamorphic rocks Pure wollastonite forms as a pyroxenoid that is triclinic due to the kinked chains
needed to accommodate all large Ca ions. If Mn is present more than a few percent, Mn analogs of
the Fe pyroxenes form (rhodonite, bustamite etc.). For other divalent cations, it requires very
unusual chemical systems around particular ore deposits to form other pyroxene type minerals. At
high pressures below 160 km depth in the mantle, Aluminum partitions into both the octahedral
and tetrahedral sites and mantle diopside from this depth is enriched in Chromium or CaTschermak’s molecule (CaAl)(AlSi))3 pyroxene .
For ease in handing in and marking the table and plot for the pyroxene analyses are moved to the
end of the pair of triangular plot exercises.
4
Representing Fe-Ni Sulphide minerals on a Triangular Diagram.
5
6
7
Name ______________________________________
Cu-Fe-S Minerals: Copper Ores and Associated Minerals
8
Name ______________________________________
MgO-Al2O3 –SiO2 : Metamorphic Minerals in Pelites (Clay rich meta-sediments)
9
Name ______________________________________
Pyroxenes
10
Pyroxenes in the CaO-MgO-FeO Triangle
11
12