Neighborhood Rocks: Saprolite

Saprolite gravel		Saprolite is a rare find in our area. We've found it in a few parking lots, and we can buy it from only a few dealers. Saprolite is also called "rotten granite" -- it really is granite that's been partially changed by thousands or millions of years of weathering. To learn more about saprolite, scroll farther down this page.

	Saprolite is mostly reddish in color, but there are also lighter colored and darker grains.
	Its crystals are usually pea-sized or smaller, and they are usually stained or coated with reddish material.
	You can probably scratch off the red coating on some grains with a nail of knife. (See more about the scratch test.)
	Because it started out as granite, saprolite is made mostly of the minerals feldspar and quartz, and may also contain small crystals of mica or darker minerals.
	Because it is now highly weathered, at least some of the original minerals have been altered to form iron oxides (like "rust") and clay.

Red granite:

	Saprolite is a weathered form of granite.
	Red granite fragments are "fresh," not stained or coated with reddish iron oxides and clay.

Pegmatite:

	Pegmatite is a granite with really big crystals (often bigger than your fingernail).
	Pegmatite fragments are "fresh," not stained or coated with reddish iron oxides and clay.

Pink quartzite:

	Pink quartzite is made of just one mineral (sand-sized quartz grains).
	Pink quartzite fragments are "fresh," not stained or coated with reddish iron oxides and clay.

Where saprolite came from

This saprolite came from a place where granite was
exposed to humid climates for thousands or millions
of years. However, we're not sure exactly where that
was. Saprolites are most common in the southern
United States, but they are also found in some
Midwestern states, like Wisconsin and Minnesota.

Since we live within a few hundred miles of an active
"rotten granite" quarry in Wisconsin, we're wondering
if that might be the source of our local supplies.

How saprolite formed

Saprolite forms when granite and other igneous and
metamorphic rocks are exposed to humid climates for
thousands or millions of years. As the rock weathers
in place, some of the original minerals, like mica,
feldspars, and darker minerals, are transformed into
new minerals, like iron oxides and clays. Quartz crystals
remain relatively unchanged -- they become quartz sand.

	We've seen 50-pound bags of saprolite gravel labeled "rotten granite."
	Sometimes we also call it "weathered granite," which is a more scientific way of saying that a rock is "rotten."

Here are some ways to classify saprolite (by grouping it with similar types of rocks):

	Our saprolite, because it is highly weathered granite, originally was an igneous rock.
	Granite is sometimes classified as an intrusive igneous rock, because it cooled and crystallized far below the Earth's surface.
	Once saprolite is extremely weathered, it might be more properly called a residual material.

Links to Web sites about saprolite

Saprolite in Minnesota. This page describes a saprolite that
may have formed during dinosaur times, and then was buried
and preserved for millions of years.
< http://www.geo.umn.edu/mgs/stearns/corestone.html >

Geology of the Bathroom. When granite goes really "rotten,"
the feldspars weather to form kaolin or "china clay." If you want
to find out what's made from kaolin, visit this site:
< http://www.bbc.co.uk/education/rocks/bathroom.shtml >

Neighborhood Rocks Search this Site

Saprolite ("SAP-row-light")

How to recognize saprolite

Other rocks that look like saprolite

Where saprolite came from

How saprolite formed

Other names for saprolite

Links to Web sites about saprolite