Cleavage has to be one of my all-time favorite geology words. I love that I’m able to use and hear the word cleavage regularly in my work. I mean, wouldn’t you be fired or at least put on probation for excessive use of the word cleavage in many lines of work?
When I’m in the field, I use the word cleavage at least daily– probably closer to hourly. When I’m working on mineral separates, I use the word cleavage much more often than that. Maybe even a few times a minute, depending on the topic of conversation. I want to thank the geologist(s) who decided to use this word to describe not just one but TWO geological properties. This doubles the numbers of opportunities in which I (and other geologists or people talking about geology) can use the word cleavage as part of a technical, no-nonsense, completely work appropriate conversation.
Actually, I must admit that I have become somewhat jaded by my frequent use of the word cleavage. When I first heard the geologic use of the word in a high school science class, I snickered along with the rest of the class. I found the term hilarious, especially because the teacher and class kept using the word. These days, I often use the word without even noticing. On second thought, I’m not completely jaded. Cleavage is still funny… just as orogeny and schist are still funny. Being a geologist is fun!
According to my New Penguin Dictionary of Geology (First Edition, 1996), cleavage has two geologic definitions:
“1. A mineral property whereby it breaks along regular, crystallographic planes.”
For instance, cleavage planes are visible in this picture of calcite I poached from google.
“2. A foliation formed by deformation at low metamorphic grade, along which rock splits preferentially.”
For example, in this picture of an outcrop I also poached from google.
As a word of advice, when looking for photos of geologic cleavage on google, be use to use search terms such as “cleavage mineral” or “cleavage geology.” Don’t just search google images for “cleavage.” Trust me, it will be a long time before an image of mineral cleavage shows up. Page nine, actually, for my search. Interestingly, an image of boob cleavage shows up on page two when you google image search “cleavage geology.” Try it. Intriguing.
I was thinking a fair amount about cleavage– the mineral kind- today, actually. I spent most of this afternoon working on a pyroxene mineral separate from one of my peridotite rocks from Oman. I sawed, crushed, and sieved the sample last week. I also used a franz (a magnetic separator) to obtain several different magnetic fractions. Since pyroxene is less-magnetic than some of the other minerals (oxides, olivine, serpentine) and more magnetic than some (carbonate) in my partially-altered peridotite, magentic separation is a good technique for separating out pyroxene. After magnetic separation, hand picking with tweezers under a microscope is necessary to ensure that there is really a pure fraction. When using the magnetic separator, some grains can jump into the wrong container. Also, bimodal and altered grains that are not pure pyroxene may end up in the pyroxene fraction. Magnetic separation basically just reduces the amount of time you need to spend picking out pyroxenes– but hours of picking are still necessary, at least for the samples I’m working on now. One of the ways I’m able to identify the precious pyroxenes is by their distinctive cleavage– when I crushed the rock, the pyroxenes fractured in a distinctive manner.
A few days ago, a researcher with whom I’m working looked at some of the pyroxenes with me to help me identify them in my separate. Here’s a snippet of our conversation. I’ve fictionalized the conversation somewhat as I don’t remember the exact wording, but this is fairly close to the actual conversation. To maintain anonymity, I will refer to the researcher as Dr. Cleavage. Just, you know, so I can type the word cleavage a few more times.
Evelyn walks into Dr. Cleavage‘s office, balancing a tray of mineral fractions.
Evelyn: Hey, Dr. Cleavage.
Dr. Cleavage: Oh– hi, Evelyn.
Evelyn: Is this a good time? I can come back later if you’re busy.
Dr. Cleavage: No, no. Just checking email. How are those mineral separates going? Looks as if you have a few fractions.
Evelyn: Yes. Would you like to take a look?
Dr. Cleavage: Is there any good cleavage?
Evelyn: Yeah, there’s some great cleavage. Do you want look under the microscope?
Dr. Cleavage: Sure. Let’s see if we can tell apart the pyroxene cleavage and the serpentine cleavage.
Evelyn and Dr. Cleavage walk over to the microscope and pour out some mineral grains on a sheet of weighing paper.
Evelyn: I think the cleavages look somewhat similar, but the orthopyroxenes have a distinctive yellow color.
Dr. Cleavage: Yes, color is probably most distinctive. But do you see the sharper cleavage in some of these pyroxenes? The serpentine cleavage is more rounded. Serpentine doesn’t have strong cleavage.
Evelyn: I think so. Are those green ones you’ve set aside clinopyroxene?
Dr. Cleavage: I believe so– take a look at the cleavage. Yes… yes, the cleavage is definitely pyroxene. Looks as if you’ll have two pyroxene fractions. That’s great! See how helpful cleavage is?
You get the idea… so, how about the skepchick readers? What’s your best (geologic) sentence containing the word cleavage? Better yet, a challenge: use the word cleavage at work or school in the near future. Try using it a vague sentence such as “Wow, I saw some great cleavage this weekend.” When your co-worker or classmate looks shocked, continue: “Yeah, I was at this rock and mineral show…” Let me know how it turns out.