Edumaction, I Gotted Some
Mar. 24th, 2008 03:27 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
affrecaSo, this guy decided to read the OED and write a book about it. And then write some blog posts, including one about absurd entries. Oddly enough, I mostly understand the definition for trondhjemite* - 'Any leucocratic tonalite, esp. one in which the plagioclase is oligoclase’. As you can guess from the "ite" ending, it is a geologic term (a rock).
Leucocratic is just a fancy geologic term for "light colored", but is only applied to igneous rocks. It is a term that you apply to a hand sample. Generally, it means that the rock is felsic (a term derived from felspar and silica), or rick in quartz, feldspars or felspathoids (types of minerals with a high silica content).
Tonalite is a type of plutonic (solidified underground, visible crystals) igneous rock. Like granite, it has a high percentage of quartz (20-60%) and feldspar. However, 90% of the feldspar is plagioclase.
Feldspars have three end members, depending on the cation. Na and Ca are of similar size, so easily substitute for each other in the crystal matrix, and thus can exist in the same crystal (K is big enough that it usually distorts the matrix so it cannot coexist in the same crystal in large amounts). A feldspar of with mostly Na and Ca is called plagioclase.
Plagioclase is further divided by the amount of Na compared to Ca (measured as % anorthite, with anorthite being plagioclase with all Ca). Oligioclase is plagioclase with 10%-30% anorthite.
So trondhjemite is a light colored rock, with visible crystals of quartz (20-60%), Na rich feldspar and minor amounts of dark minerals like biotite. (roughly).
*Wikipedia has two articles for trondhjemite, one of which is misspelled. But they're refusing to let me log in, so I can suggest a merger/deletion). ETA now fixed.
**Moonstone is a type of K rich feldspar. Labradorite is another mineral used in jewelry, and is a plagioclase with 50%-70% anorthite. It's shiny.
Leucocratic is just a fancy geologic term for "light colored", but is only applied to igneous rocks. It is a term that you apply to a hand sample. Generally, it means that the rock is felsic (a term derived from felspar and silica), or rick in quartz, feldspars or felspathoids (types of minerals with a high silica content).
Tonalite is a type of plutonic (solidified underground, visible crystals) igneous rock. Like granite, it has a high percentage of quartz (20-60%) and feldspar. However, 90% of the feldspar is plagioclase.
Feldspars have three end members, depending on the cation. Na and Ca are of similar size, so easily substitute for each other in the crystal matrix, and thus can exist in the same crystal (K is big enough that it usually distorts the matrix so it cannot coexist in the same crystal in large amounts). A feldspar of with mostly Na and Ca is called plagioclase.
Plagioclase is further divided by the amount of Na compared to Ca (measured as % anorthite, with anorthite being plagioclase with all Ca). Oligioclase is plagioclase with 10%-30% anorthite.
So trondhjemite is a light colored rock, with visible crystals of quartz (20-60%), Na rich feldspar and minor amounts of dark minerals like biotite. (roughly).
*Wikipedia has two articles for trondhjemite, one of which is misspelled. But they're refusing to let me log in, so I can suggest a merger/deletion). ETA now fixed.
**Moonstone is a type of K rich feldspar. Labradorite is another mineral used in jewelry, and is a plagioclase with 50%-70% anorthite. It's shiny.
