Mineral Physics Institute Summer Scholars Program

Marco Finotello Mount Union College, OH Major: Geology Mentor: Hélène Couvy

Summer Project Description:

Title: Relative Strength of Olivine and Wadsleyite Using a Structural Analogue (Mn 2 GeO 4 )

Olivine, (Fe,Mg) 2 SiO 4 , is one of the most abundant minerals from the Earth's surface down to the transition zone.
At the pressure and temperature of the transition zone, it transforms into two high pressure polymorphs, wadsleyite
and ringwoodite. The pressure and temperature at which wadsleyite is formed is very high, roughly 14-20 GPa and 1400°C.
The investigation of rheological properties of this phase is limited by the current technologies. As first approximation,
in order to study minerals within their stable range, a structural analogue, Manganous Germanate (Mn 2 GeO 4 ), can be used.
While chemically different, the analogue presents two phases structurally identical to olivine and wadsleyite
{(Morimoto, 1969 #61; Wadsley, 1968 #451)}, allowing for the lowering of the pressure and temperature when conducting
the experiment. The alpha ( ) phase of the manganous germanate is a structural analogue of olivine while the beta ( ) phase
is the structural analogue of wadsleyite.

In this project I will be studying the rheological properties of Mn 2 GeO 4 with the hopes of better understanding the
mineral's properties from the Earth's surface down to the transition zone. These experiments will be performed by deforming
a composite sample consisting of either a or b -Mn 2 GeO 4 and a standard material, which can be compared with the analogues.
The deformation conditions will be kept similar for each experiment. The relative strength of Mn 2 GeO 4 polymorphs will be
studied at 1000°C and at 3 and 5 GPa , respectively. The standard will be chosen among materials which do not change
substantially of strength over the pressure range studied. Moreover, this standard will provide a stress buffer in order to apply
about the same deviatoric stress level in these experiments. The strength contrast can be assessed by measuring the strain and
strain rate of each Mn 2 GeO 4 polymorph and the standard material as well as from the strength of the fabric produced.

In course of the REU program, I will be studying the alpha phase which will later be compared to that of the beta phase.

Fig 1. Experimental setup at the X17B2 beamline of NSLS

(After Chen et al. 2004)

Reference: Chen et al., Deformation experiments using synchrotron X—rays in situ stress
and strain measurements at high pressure and temperature, Physics of the Earth and Planetary Interiors, 143-144, 347-356, 2004