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Identity HelpWhat causes colours in gypsum (satin spar)?

8th Jul 2017 08:32 UTCDennis Jones

Yesterday my field trip to West Bay, near Parrsboro, Nova Scotia, Canada was eventful in many ways (I won't mention the bad ones). That being said, I discovered a new colour of satin spar gypsum - yellow! I have several colours including white, grey, black and orange, but yellow was a complete surprise. In fact, finding gypsum at West Bay was a surprise in itself as I have never seen it mentioned for this location in the literature.

Here is a picture of a nice yellow piece in its current habitat. I asked myself, and now the mindat community, why would this piece be yellow? Or others orange for that matter?

Cheers, Dennis

8th Jul 2017 12:21 UTCDennis Jones

Here are samples of orange and black gypsum satin spar in Cheverie, Nova Scotia.

8th Jul 2017 12:34 UTCDennis Jones

White and orange satin spar gypsum from Cape Blomidon, Nova Scotia.

8th Jul 2017 12:40 UTCDennis Jones

Gray satin spar gypsum from Cheverie.

8th Jul 2017 13:00 UTCDennis Jones

I have some thoughts about the role of iron in some of the coloration of satin spar gypsum. After I have my morning coffee I'm going to draw a map which I'll post here.

8th Jul 2017 14:04 UTCBob Harman

DENNIS, You are correct. One of the very many reasons minerals have various colors is from iron staining. Rusty iron oxide staining gives gypsum various shades of yellow, orange and brick red colorations. Some examples are aesthetically colored (stained) while many other examples are just dingy and stained. In some cases the coloration is nearly completely on the mineral surface creating a sort of crust, while in other cases the iron coloration does extend a bit into the mineral, or at least into micro hairline spaces and fractures on the surface. At least a few of your examples appear to show some iron staining while the color in your other examples is not so straight forward. Identical situations exist for many minerals, common among them is iron stained quartz, calcite, dolomite, fluorite, and most other common minerals.

I am sure other responders will give many additional reasons minerals are variously colored. CHEERS.....BOB

9th Jul 2017 00:54 UTCDoug Daniels

I will agree for the most part with Bob - it's due to various iron oxides, either as staining or incorporated in the specimen. Depends on the specific mineral, and the concentration. Danged iron, but, where would we be without it?

9th Jul 2017 01:13 UTCDennis Jones

Thought so. I went through my boxes of minerals from all three places with a strong magnet and detected iron in specimens. Both Cape Blomidon and West Bay have magnetite. In Cheverie the magnetic attraction is weaker but is associated with materials in the gray clay for the most part. I may be in Cheverie tomorrow so I'll bring some magnets with me.

9th Jul 2017 02:18 UTCDoug Daniels

Just remember, not all iron minerals are magnetic. Magnetite, well duh. Pyrrhotite, a sulfide is another (you likely don't have that). There's another oxide, magheimite (I think, spelling may be off) that can be magnetic. None of these tend to be associated with gypsum though.

9th Jul 2017 10:01 UTCDennis Jones

Thank you Doug and Bob. You guys are special!

Dennis

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