FMF - Friends of Minerals Forum, discussion and message board

[Jim Prentiss]

Hello Ladies and Gentlemen,

I am requesting either opinions or enlightenment.

Some years ago I had read an article and jotted down some of the particulars in my notes and promply forgot about it. This afternoon I reread them and the author, whom I did not note, had stated that Colloform emcompasses botryoidal, tuberose, globular, nodular, spherical, mammilary, kidney and reniform. I have for some time tried to nail down some specific criteria for some of these and I thought I had it close. This now would negate all that.

So, is the mineralogical community in aggreement, and thus we can simplify some descriptions, or is there really technical differences between these terms?

Yours in ignorance,

Jim Prentiss

[Jesse Fisher]

Jim,
My copy of the McGraw-Hill dictionary of Geology & Mineralogy (1994) defined colloform as "Pertaining to the rounded, globular texture of a mineral formed by colloidal precipitation." This implies a specific method of formation for the mineral, which the other terms do not, so I would not think them synonymous, unless all minerals that display such rounded morphologies form by colloidal action.

[Pete Modreski]

Jim,
Finding a slight modification of the definition that Jesse quoted, the Glossary of Geology (current edition) says,

"colloform: rounded, finely banded kidneylike mineral texture formed by ultra-fine-grained rhythmic precipitation once thought to denote deposition of colloids.

also,
"botryoidal: Having the form of a bunch of grapes. Said of mineral deposits, e.g. hematite, having a surface of spherical shapes;
"mammillary: Forming smoothly rounded masses resembling breasts or portions of spheres."
"reniform: Kidney-shaped. Said of a crystal structure in which radiating crystals terminate in rounded masses"

Overall, as you say, I think these terms are rather vaguely defined and it's hard or impossible to precisely discriminate between them. I think that colloform is most often used when the spherical mineral aggregates appear to be composed of fine-grained particles, as opposed to the other terms that are used when the material appears more fibrous and crystalline. The fine-grained examples look like they could have been deposited as colloidal particles, but I think now people are hesitant to claim that they really necessarily formed that way--hence the qualification in the definition. When the mineral is one of the varieties of carbonate-apatite, people tend to use "colloform"; when it's fluorite, hematite etc., then botryoidal or one of the other terms. I think the usage is much determined just by "tradition", and by the size of the rounded masses--mamillary, large; botryoidal, smaller, or generically of any size. People just don't tend to use "reniform" all that much these days.

Pete

[Jim Prentiss]

Hello Jesse and Pete,

Sorry for taking so long to get back to you guys, I have been off the net for a bit.

I was suspicious of the article when I read it and that is why a made a note to follow up (albeit about ten years later), it seemed too simple. Mineralogy needs to be more descriptive than what that fella was proposing.

Thanks again,

Jim Prentiss