INTRODUCTION TO QFL
QFL stands for Quartz, Feldspar, Lithics


Sedimentary rocks are classified on the basis of the texture (grain size) of the rock, and composition.
The basic classification only concerned texture, using the Wentworth size scale. But any full rock name must specify both texture and composition. Thus, an arkose sandstone is a rock of sand sized particles, with a high percentage of those particles being feldspar. It might seem that an unlimited variety of particles could end up in a sedimentary rock. After all, there are over 6000 known minerals. In addition, any incompletely weathered piece of igneous, sedimentary, or metamorphic rock can also be found in a sedimentary rock. A composition classification could become very complicated if all of these different particles were considered. But in most cases rock composition can be defined by four compositional components:

    • Quartz
    • Feldspar
    • Lithic fragments (including rock fragments and mineral grains other than quartz)
    • Matrix (a catchall for the silt and clay grains that cannot be easily seen by eye).
COMPONENTS OF SILICICLASTIC SEDIMENTARY ROCK COMPOSITION
Quartz Since quartz, for all practical purposes, does not weather into anything else, and will remain after everything else is weathered or sorted out, it is one of the most important of the four components of sedimentary rock composition.
 Pure quartz sandstones are rare. Usually quartz is mixed with one or more of the remaining three components. Pure quartz sandstones form only under great tectonic stability when the land is not high enough for rocks to be exposed to weathering.
Feldspar Feldspars are some of the most abundant minerals in the earth's crust. With only a few exceptions all igneous rocks have large amounts of feldspar; e.g. calcium plagioclase in gabbro, and sodium plagioclase and orthoclase in granite. Medium to high grade metamorphic rocks also have large amounts of feldspar.
 Sediments near high mountains frequently have large percentages of feldspar as batholiths and regional metamorphic rocks are uplifted and eroded. Rifts frequently also have large amounts of feldspar.
Lithics Very simply, if a siliciclastic particle is not quartz or feldspar it is classified a lithic fragment. Lithic means "rock" and all mechanically weathered pieces of another rock, or non-feldspar minerals weathered from a rock, are included here. Frequently they are small, dark in color, and difficult or impossible to specifically identify in hand specimen. The exception to this is conglomerates and breccias. Lithic fragments are especially abundant in volcanic arc systems but are common in most collision, mountain building areas.
Matrix Matrix is the finer material in which larger particles are embedded. So, in a sandstone the matrix is silt and clay. In a gravel the matrix may be a sand. However, since all minerals other than quartz will eventually weather into silt or clay sized particles, silt or clay is very common in sedimentary rocks.


QFL CLASSIFICATION AND IDENTIFICATION SYSTEMS

A Ternary System for Clastic Sedimentary Rock Identificaton   

The universal system used by geologists is based on the ternary diagram. This classification requires two ternary diagrams, one for determining texture, a second for determining QFL. These diagrams are discussed separately below, but are also combined into one diagram. And sometimes we need a chart to help estimate percent abundance.

QFL Diagram:

The QFL diagram is to the right. Observe the following:

Quartz is at the top, feldspar on the lower left, and lithics on the lower right. It is always done this way.

The ternary diagram is divided into 5 fields, here color coded. The boundaries among the fields, left and right, are at the 50% boundary, and up and down at the 75% and 90% boundaries.As you travel toward any apex the quantity of Q, F, or L increases accordingly, with 100% being, of course, right at the apex.

Notice that as we travel vertically the amount of quartz in the rock increases, and at the 90% boundary and above the rock has so much quartz the rock becomes a "quartz something", such as a quartz sandstone or quartz conglomerate.

The lower two fields contain rocks that are felspar (red) or lithic (blue) rich. That is, these rocks have more than 25% feldspar or lithics, that is, 25-100% feldspar or lithics. Rocks with this composition have such names as feldspathic (arkosic) sandstone (both terms are used interchangably) and lithic sandstone.

Remember that all feldspar and lithic fragments are going to weather and disappear (to shale or dissolved minerals), leaving only quartz. On the QFL diagram, however, we can only plot the abundance of sand (or larger) particles of various compositions. So, on this diagram, as feldspar and lithics weather the composition of the remaining sandstone migrates toward the quartz apex. No matter where you start on the diagram the sediment is going to evolve in almost a straight line right to the top. One of the things we are very interested in is how close the sediment has progressed along its path of evolution. This is the concept of sediment maturity. Thus, above the arkosic and lithic fields, but below the quartz field are two more fields, subarkosic and sublithic. Rocks in these fields have between 10-25 % feldspar or lithics and are thus farther along in their evolution toward pure quartz than feldspathic or lithic rocks.

In other classsification systems, the boundaries among the fields sometimes differ from this one, and there may be more fields than 5 laid out. It all depends on what the geologist wants to do with them. But for this site we will always have these five fields, in these five places.

Observe that a composition plotted somewhere in the middle of the QFL indicates a mixed composition. For example, the composition of "A" to the right is about 50% quartz, 35% feldspar, and 25% lithics (ternary with percent numbers). We could just call it an arkose since it falls in the feldspar field, but it would be more accurate to indicate that a lot of lithics are present too. Such a name, following the rules for naming rocks is lithic, feldspathic, quartz.


 

 

Texture Diagrams:

The basic texture diagram is to the right. Observe the following:

This diagram is also divided into five fields, but the percentage cutoffs differ from the QFL diagram.

The apexes are sand, silt, and clay. Where QFL apexes remain constant, texture apexes commonly change for different uses. Always check the apexes. Later we will use some different apexes to explore some ideas.

WACKE :-). Sandstone, siltstone, and shale are easy to identify. And even a sandy shale or sandy siltstone is not bad. But the shaley sand and silty sand fields, in practice, are difficult or impossible to distinguish from each other, even under a good microscope. Historically, these have been put together under the name "wacke".



If a rock has gravel sized particles then we need a ternary diagram with gravel at one apex, as to the right. The fields have also changed boundary conditions.
Observe here that sand has moved to another apex, and that silt and clay have been combined into one category, matrix.

   Go to all ternary diagrams combined into one diagram.

To name a clastic sedimentary rock we must plot the composition on the QFL diagram, and the texture on the texture diagram, and then combine the two to get the full name. See example below. for a specimen plotted at "A"

 

A couple of simplifying rules.. If quartz is the most abundant composition it can usually be left out of the name; it is implied then that quartz is present and most abundant. However, if it is not the most abundant it must be included in order of its abundance. And, when a composition is less than 10% we often leave it out too. AND a practical consideration. How do you learn how to estimate percentage abundances? The most common method is to use a percent abundance chart.

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