🔎 UNIDENTIFIED Possibly arkose?

[Blackfoot58]

The photos show 2 rocks. My main interest is the larger one. These are common field finds in my part of Iowa. I believe the small one may be magnetite. Both rocks have iron content and hold a magnet easily.
The larger one has a much higher density. I’ve looked at many on-line sources. Magnetite or hematite pop up often. To me, none of the examples look like this one.

To me, it resembles granite, in mass/weight, crystal content and coloration. I’m not thinking any value. I just would like to do a positive ID.
Thanks for any help.👍🏼

 

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[fuss]

The two look like the same rock. You should be able to get a streak color from the black minerals if you apply enough pressure, red=Hematite, black=Magnetite.

If you dont already have one get a 10x loupe so you can take a close look at the individual minerals, will be very helpful when trying to discern all the varieties of glacial till rocks.

 

[ToddsPoint]

Gabbro maybe?

 

[Blackfoot58]

The two look like the same rock. You should be able to get a streak color from the black minerals if you apply enough pressure, red=Hematite, black=Magnetite.

If you dont already have one get a 10x loupe so you can take a close look at the individual minerals, will be very helpful when trying to discern all the varieties of glacial till rocks.

They aren’t the same. The smaller one is much less dense. It is 1.6 oz
The larger is 18.4 oz. Which is 11.5 times heavier, but definitely not 11.5 times more volume. I do have a good eye loupe. The large rock has crystal structure quite evident; the smaller one does not. Thanks👍🏼

 

[Blackfoot58]

Gabbro maybe?View attachment 2147828

I wondered about that. It is in our area, but I couldn’t find any cases of it having high iron content.

 

[Blackfoot58]

The two look like the same rock. You should be able to get a streak color from the black minerals if you apply enough pressure, red=Hematite, black=Magnetite.

If you dont already have one get a 10x loupe so you can take a close look at the individual minerals, will be very helpful when trying to discern all the varieties of glacial till rocks.

I scratched unglazed porcelain with both. The small one left a gray line. The large a darker gray/black.

 

[Clay Diggins]

There is no streak test for rocks. Streak tests are only useful on relatively pure mineral specimens.

Rocks are composed of several minerals, that's why they are called rocks and not called minerals.

Some Gabbro is attracted to a magnet but so are many other minerals.

If a rock is common in your area your state geologist will have maps showing trhe predominant rock types in your area. That would be a good place to start when looking for a rock id.

Even easier is to go to this map:

Iowa High Resolution Geology

Land Matters

www.mylandmatters.org

zoom in to your area of interest, click on the "i" tool then click on the map and you will get a list of all the many geology maps available for your clicked area. Download directly from the National Geologic Map Database. You will find your rock there.

 

[Blackfoot58]

There is no streak test for rocks. Streak tests are only useful on relatively pure mineral specimens.

Rocks are composed of several minerals, that's why they are called rocks and not called minerals.

Some Gabbro is attracted to a magnet but so are many other minerals.

If a rock is common in your area your state geologist will have maps showing trhe predominant rock types in your area. That would be a good place to start when looking for a rock id.

Even easier is to go to this map:

Iowa High Resolution Geology

Land Matters

www.mylandmatters.org

zoom in to your area of interest, click on the "i" tool then click on the map and you will get a list of all the many geology maps available for your clicked area. Download directly from the National Geologic Map Database. You will find your rock there.

Thank you! I was unaware of this.👍🏼

 

[fuss]

Hematite streaks red, so the rock likely has Magnetite which leaves a dark streak, Pyroxenes streak can be grayish but usually white, Hornblende white grey.

So Gabbro can have Magnetite, Hornblende (Amphibole), Pyroxenes along with the light colored Sodium Feldspar in them so I think Gary had a good guess at what it might be.

And yes streaking rocks can be pointless most of the tome but you can do it if the minerals are large enough and just trying to differentiate between magnetite and Hematite.

 

[Blackfoot58]

Hematite streaks red, so the rock likely has Magnetite which leaves a dark streak, Pyroxenes streak can be grayish but usually white, Hornblende white grey.

So Gabbro can have Magnetite, Hornblende (Amphibole), Pyroxenes along with the light colored Sodium Feldspar in them so I think Gary had a good guess at what it might be.

And yes streaking rocks can be pointless most of the tome but you can do it if the minerals are large enough and just trying to differentiate between magnetite and Hematite.

That’s good info. Thank you. It does appear to have what I believe is feldspar.