iron. The results of these experiments and those of previous experiments conducted in graphite are
strikingly similar: in both cases the residual liquids were depleted in silica and enriched in Fe, Ti, and
P. We conclude that the presence of carbon is not essential for producing this distinctive trend.
Keywords: Experimental petrology, tholeiite, crystal-liquiddifferentiation, phase equilibria
al. (2007). Experiments in this study were conducted in Pt capsules that had been
pre-soaked with Fe° foil to prevent net transfer of Fe between sample and capsule
SOME CHEMICAL PROPERTIES of FELDSPARS
J. V. Smith
This chapter considers some chemical features of feldspars which show
promise for interpretation of their genesis in terms of rock-forming pro-
cesses. Of course, the volume percentage and major element content of feld-
spars are used for petrographic classification of many rocks in the crust of
the Earth. Furthermore, the major element content of feldspars has petrogene-
tic significance, especially with regard to the overall trend of crystal-
liquiddifferentiation from calcic to
some terrestrial and lunar basalts, is a fine-
grained mixture of calcium-rich pyroxene and plagioclase that
formed on the surface of its parent body; like some terrestrial basalts,
it reflects rapid crystallization with very little, if any, crystal-liquiddifferentiation. Some eucrites and the sherghottite shown in the bot-
tom photograph are coarser-grained and reflect slower crystallization
at greater depths in their parent bodies.
The two rocks in Figure 7.3 are so similar in mineralogy and chemi-
cal composition that earlier researchers lumped eucrites and
12 chemical groups. Some of these groups
overlap in Figure 8.8, but they are distinct in diagrams for other
elements. In naming these iron groups, Wasson and his co-workers
used the Roman numerals that Lovering had devised, adding letters to
subdivide the four original groups. In cases where two groups ap-
peared to be related, they showed this by using two letters. Thus
groups IIIA and HIB, which are similar in many respects and appear
to intergrade, appear as the field IIIAB in Figure 8.8.
Because there is no obvious way for crystal-liquiddifferentiation
the ultimate goal of the differentiation process as leading toward the
production of eutectic mixtures.
In his study of the igneous rocks at Magnet Cove, Arkansas, H. S. Washing-
ton (1900) also suggested that the differentiation might have resulted from
eutectic crystallization. He proposed the subdivision of laccoliths on the basis
of their rock types and the nature of their differentiation. Laccoliths of the
Henry Mountains type, he suggested, might represent magmas that crystallized
as nearly eutectic mixtures.
A second proposed mode of crystal-liquid