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3.3 h. Displacive phase transitions in clinopyroxenes (T. Arlt and R.J. Angel, in collaboration with M. Kunz/Zürich)

Many metamorphic, plutonic and volcanic rocks of the Earth´s crust contain the single chain silicate pyroxene as a major component. Pyroxenes also comprise some 25% of the Upper Mantle. Because pyroxenes undergo phase transitions at high temperatures and pressures, experimental determination of the behaviour of synthetic material in combination with the characterization of natural pyroxenes can provide constraints on the cooling history of the rocks in which pyroxenes occur. Thus, the discovery of the clinopyroxene pigeonite in lunar rocks obtained by the Apollo missions provided new information about the cooling history of the moon and thus constraints upon the origin of the Earth-moon system. More recently, relicts of high-pressure clinopyroxene were reported in the Alpe Arami peridotite which, if the interpretation is true, would have important consequences for the mechanisms of ultra-high pressure metamorphism of crustal rocks and their subsequent exhumation.

The polymorphism of pyroxenes is complex. At ambient conditions those with low Ca or Na-contents, such as enstatite (MgSiO3), ferrosilite (FeSiO3) and kanoite (MnMgSi2O6), have primitive space groups, monoclinic P21/c, and structures characterized by two different [Si2O6]2-chain angles. At high-temperatures as well as at high pressures, these clinopyroxenes can undergo phase transitions which are mostly displacive in nature. During the transformations the two silicate chains become equivalent and the symmetry increases to C2/c. Although the high pressure and the high temperature polymorphs display the same C2/c symmetry, the relationship between them is not fully understood. In this project we are investigating the phase transition behaviour of several clinopyroxene structures in order to gain insight into both the nature of the P21/c - C2/c phase transitions and the relationship between the high-pressure and high-temperature C2/c phases.

Our previous work on MgSiO3 and FeSiO3 showed that the silicate chains are extremely kinked in the high-pressure (HP) C2/c form while they are almost completely straight in the high-temperature (HT) form, a strong indication that the HT and HP structures are two distinct phases. A new in-situ powder diffraction study of kanoite in a heated diamond anvil cell (DAC) using synchrotron radiation has now provided evidence that the HT-C2/c structure and the HP-C2/c structure have distinct stability fields in P/T space (Fig. 3.3-4).

Fig. 3.3-4: A schematic phase diagram showing the probable topology of the phase relationships between clinopyroxenes. The origin of the phase diagram shifts in P-T space with changes in composition. The phase transitions observed experimentally in a number of clinopyroxenes are indicated by the labelled boxes.

High-pressure single-crystal X-ray studies on several compositions show that the P21/c - HP C2/c transition is first-order in character. In a new study of kanoite we have shown that at the transition pressure both phases coexist in a "single-crystal", spatially separated in the sample by a phase boundary region observed optically as a white line. We were able to measure the cell dimensions of both phases simultaneously and varied stepwise their volume proportions in the transition region. Since the small volume change imposed on the sample chamber was fully consumed by the volume change of the phase transition, the pressure remained constant during the phase transition. A possible interpretation of these observations is that the transition is martensitic and we are observing the effect of pressure buffering in the DAC. A strain energy contribution is assumed to stabilize the two phase region, breaking the classical Gibbs phase rule by providing an extra degree of freedom.

In Li-pyroxenes the phase transition between the HT-C2/c and the P21/c polymorphs is shifted to below room temperature, with the consequence that LiFe3+, LiSc3+ and LiIn3+-clinopyroxenes are reported with C2/c-symmetries at ambient conditions and may be regarded as equivalent to HT C2/c forms. Since the transition temperature increases with pressure, we performed a single-crystal X-ray study on LiSc3+-Cpx in a DAC. The h+k = odd reflections, characteristic for the P21/c symmetry, appeared at approx. 1 GPa. Non-linearities in cell parameters are significant, but much weaker than those reported in temperature dependent P21/c - HT C2/c phase transitions such as that observed in kanoite. Structure refinements at 2.2 and 4.8 GPa display different O3-O3´-O3´´ angles of the A and B chains. A similar evolution of cell parameter vs. pressure has previously been reported for ZnSiO3-Cpx in which a second, much stronger non-linearity at higher pressure indicates the transition from P21/c to HP C2/c, and we plan to study this pyroxene in more detail in the coming year.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
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