Preparation of oriented samples for mounting as petrographic thin sections

This process requires special attention to detail and meticulous record keeping so that the orientation is never lost. The objective is to produce three mutually perpendicular thin sections, one parallel to strike (PTS), one parallel to bedding (PTB), and one in the dip plane (IDP) to be mounted for large format thin sections. The inside face of the PTS block, the outside face of the IDP block, and the bottom face of the PTB block were mounted with exceptions noted in a log book.

Oriented blocks

When strike and dip data was collected in the field, an arrow was added to the side of the strike line that "pointed" north and the surface measured was recorded (under bedding surface or upper bedding surface.) This helps prevent mix ups when samples are flipped around and when the opposite mounting surface is used.

Before each cut was made a sketch of the rock and the cut being made was recorded in a log book. Orientation information (stratigraphic up, down dip direction, strike/dip symbols, mounting surface) and identification information (sample number, plane of sample) was written on each rock in at least two places with a permanent marker. Labeling the rock in several places helps prevent the loss of information when a rock is cut or if it breaks.

Mounting separates the thin section from the orientation information, so notches were cut in the corners of each block for later use as direction indicators. For the IDP block, the notch was cut in the stratigraphic up edge on the down dip side. For the PTB block the notch was cut in the right side of the down dip edge. For the PTS block, the notch was cut in the stratigraphic up edge on the left side (when viewed from top with the mounting surface down.) When exceptions were made (i.e. another corner was chosen because the corner to be cut was especially brittle or missing) it was noted in a log book.

Oriented slabs

Digital pictures were taken of each block set before packing to be shipped for mounting. Pictures were taken of the blocks oriented properly to each other with the maximum amount of markings visible [note photo above and location of notches.] This would be a record of the relationship between the blocks, notches, and mounting surfaces in case the log book was incomplete or ambiguous.

When the thin sections returned they were labeled with the sample number only. The notches were the only indicator of orientation on the thin section. For each thin section, orientation information was obtained from the log book and digital photos, then added to each thin section with a permanent marker.

Thin sections

Consistent, accurate, and redundant record keeping ensures that the sample orientation is not lost between the starting rock and the finished thin section.