Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: dinwood on 31/08/2017 07:20:45

Title: Are iron minerals often co-located with limestones?
Post by: dinwood on 31/08/2017 07:20:45

Since noticing this whilst walking in Crete some years ago, I keep seeing rusty/ochre colours associated with limestone.  Sometimes this is just a surface effect on newly exposed rock and nearby weathered rock is grey; often I see red soils above limestone (eg in the cotswolds), but I frequently see even weathered limestone stained rusty colours.  I'm guessing these colours are iron based minerals, even though the colour ranges from light orange to dark almost red. 

I keep seeing this almost wherever I go and can see what look like limestone strata - certainly all around the mediterranean and the UK (testing for limestone by the fact it doesn't scratch the back of a fingernail).  I'm no geologist but have a lifelong interest in science generally, so I hope someone can shed some light, or disabuse me of my theory. 

Title: Re: Are iron minerals often co-located with limestones?
Post by: chris on 31/08/2017 15:10:59

We need @Bass or @JimBob for this!

Title: Re: Are iron minerals often co-located with limestones?
Post by: chiralSPO on 31/08/2017 16:04:11

I will defer to any geologist on this answer, but as a chemist I will say:

Iron is very common, and there are many different types of iron-rich minerals, not all of which are strongly colored in the recognizable reds, oranges and yellows. It could be that the higher pH (more alkaline) of regions containing significant amounts of limestone (CaCO₃) favors these more colorful iron minerals.

There may also be a historic reason for the apparent colocation. Many limestone deposits are mineralized exoskeletons of ancient marine organisms, and I believe that a significant portion of the deposits were formed at a time that iron was precipitating out of the ocean because of biogenic oxygen production (photosynthetic organisms produced oxygen in the oceans, which reacted with the highly soluble Fe²⁺ ions, oxidizing them to less soluble Fe³⁺ ions.)

Title: Re: Are iron minerals often co-located with limestones?
Post by: Bass on 30/09/2017 22:03:00

Limestones are carbonate rocks- with a wide range of mineralogy. On a worldwide scale, calcite (calcium carbonate) is the most abundant carbonate mineral in limestones, but dolomite (magnesium carbonate), ankerite (calcium iron carbonate) and siderite (iron carbonate) are also common, and may be more or less abundant on a local scale. Since most limestones contain at least some of these other carbonate minerals, they commonly show iron colored weathering. In my experience, an orangish-brown color is the most common in weathered limestones, which may become more reddish brown if siderite is more abundant.