Cleavage isn't just for diamonds: my field mistake

I used to think rock cleavage was not that important for ID. I would just look at color and feel the weight. Then I picked up what I thought was shale, but it split into perfect flat sheets. A buddy told me it was slate, and cleavage was the key clue. Now I check for cleavage right away, and it helps a lot. For instance, last week I spotted phyllite fast because of its shiny layers and how it broke. Changing my mind on this made my rock collecting much better.

3 comments

3 Comments

nora_wood433mo ago

I identified mica as glass once. Cleavage matters.

elizabetht563mo ago

So when you figured out it was slate... did that make you look at other layered rocks differently? I got hung up on foliation for ages, thinking all flat breaks were the same. But that shine on phyllite versus slate's dull sheen... it's a whole other clue once you start looking. Makes you wonder how many rocks we group together that split on totally different planes.

juliaa252mo ago

My first geology trip I called a piece of calcite quartz because it was clear. It cleaved into rhombs right in my hand and I felt so dumb. @elizabetht56 you're right about the shine being a giveaway too. I spent a whole afternoon trying to split a sandstone like slate before I got that not all flat planes are foliation. That calcite moment was my real wake up call.