Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
First-time Visitors: Please visit Site
Map and Disclaimer. Use "Back" to return here.
One of my most vivid field trip memories is encountering a dazzling white
wollastonite marble in the desert in California. It had huge spears of
wollastonite and was so tough sledgehammers simply bounced off it. (It's no
longer collectible, alas, since it's inside the expanded boundaries of Death
Valley National Park.)
Since calcium is a large cation, it won't fit into the M1 sites of a standard
pyroxene. The only way to accomodate silica chains to it is to kink them
differently. The chains consist of scalloped chains with a repeat interval of
- A conventional pyroxene chain with cation
octahedra. The medium-sized cation forms an octahedron small enough
for pairs of tetrahedra to span.
- If the cation is calcium,
the octahedra are too big for silica tetrahedra to link.
- A side view.
- The only way to link tetrahedra across the Ca-O
octahedron is to cant them toward each other.
- Canting the
tetrahedra inward allows pairs to link up, but then there is no way
to link the pairs.
- We can form chains by alternating
mirror image pairs of tetrahedra, but note that the chains now cut
obliquely across the chains of Ca-O octahedra, as shown below. The
structure is made up of en echelon segments of Ca-O chains, or three
parallel chains of octahedra joined along opposite edges.
Below: a view of wollastonite perpendicular to the silica chains. Yellow denotes Ca-O octahedra, and pink and purple signifies silica tetrahedra.
Below: end view of the wollastonite structure
Return to Thin-Section Index
Return to Crystal Structures Index
Return to Mineral Identification Tables
Return to Professor Dutch's Home Page
Created 22 April 2013, Last Update
05 Jun 2013
Not an official UW Green Bay site