Monthly Archives: June 2019

The mineralogy of sandstones: lithic fragments

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Identifying detrital lithic fragments

This post is part of the How To… series

Lithic fragments are the bits of eroded or broken rock that can’t be easily slotted into either the quartz or feldspar classification end-members. They are the fragments that are not broken down into single minerals. They tend to be fine-grained and rather dirty looking in shades of brown and grey. In thin section they are nowhere near as exciting to look at as other framework grains. But If we want to know something about sediment source rocks (provenance) or the longevity and survival of granular sediment during transport and deposition, then lithics are no less valuable than quartz, feldspar – perhaps more so.

R.H. Dott’s classification divides the lithic end-member into sedimentary, volcanic, and metamorphic clasts; R.L. Folk takes each of these a step further (I’m not convinced Folk’s subdivision is practical). Part of the problem with too fine a subdivision is the diagenetic and mechanical alteration that lithics are prone to, rendering them indeterminate. Lithic fragments are much softer and chemically more reactive than their quartz-feldspar counterparts during burial diagenesis. Continue reading

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The mineralogy of sandstones: feldspar grains

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Identifying detrital feldspar.

This post is part of the How To… series

Quartz may be the most common mineral in sandstone, but feldspar is the most abundant mineral in pretty well every other rock type; in fact, it is the most abundant mineral in the Earth’s crust. Unlike quartz, feldspar is an essential ingredient in nearly all igneous rocks, felsic through ultrabasic. It begins to crystallize in magmas at temperatures about 1000oC – 200o warmer than quartz crystallization. Feldspars are also common in metamorphic rocks. As such, feldspar is an important (usually subordinate) component of most terrigenous clastics, reflected in its inclusion in QFL classification schemes.

The two major groups of feldspar are potassium feldspars and plagioclases. All have low relief in plain polarized light (similar to quartz). Both groups have two good cleavage planes at 90o to each other such that broken crystal fragments tend to be blocky. Twinning is common. Continue reading

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The mineralogy of sandstones: Quartz grains

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This post is part of the How To… series – quartz mineralogy in sandstones

Classification of terrigenous sandstones depends on the identification of two main components: framework grains and matrix. Frameworks are represented by a QFL triad – quartz, feldspar and lithic fragments, where the proportion of each grain type is determined from thin section.  Most classification schemes aggregate all types of quartz, feldspar and lithics into each end-member. This approach is sensible and easy to use.

But simply naming a sandstone (or any rock type for that matter) is not enough. We also want to know about its provenance, the sediment source or sources – was it a stable continent or active mountain belt, volcanic arc or ocean basin, perhaps a far-travelled terrane or tectonic sliver for which the only evidence is the collection of grains that have survived multiple cycles of attrition.

Teasing this information from the rocks requires us to delve into the mineralogy in greater detail. The simplest and cheapest way to do this is with thin sections and a polarizing microscope. We begin with the most common terrigenous component – quartz. Continue reading

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Classification of sandstones

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sandstone classification

This post is part of the How To… series – how to classify sandstones using QFL plots

In science, classification of things is one of those tasks we readily identify as a crucial component of knowledge but prefer that someone else does it. Classification schemes don’t just name things, they organize them according to their properties, appearance, structure, composition.  If I wish to talk about a particular rock or fossil, then the people who are interested in such things will have a frame of reference to understand and contribute to the discussion, based on whatever classification scheme applies.

The classification of sandstones matured in the 1940s through 1960s; many publications were devoted to the subject; some of the key players were Robert Folk, Harvey Blatt, Francis Pettijohn, Raymond Siever, P. Krynine, E. McBride, H. William, F Turner, C Gilbert, Robert Dott, and R. Fisher. Several schemes were proposed and debated; few were accepted. One of the central topics of discussion was the relative importance of sandstone texture versus sandstone composition. A classification based on texture alone was deemed inadequate; if the rock or sediment had >50% sand, then it was a sandstone, or arenite. Qualifications such as pebbly, silty or muddy might be applied, but this said nothing about the variability of mineral types. However, textural properties such as the percentage of matrix (clay plus silt) did provide grounds for distinguishing between ‘clean’ sandstones (i.e. those lacking significant matrix) and wackes – those rocks containing significant matrix. Continue reading

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