geology · petrology
QFL Sandstone Classification (Folk, 1968)
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Inputs
Paste one row per line — separate columns with a comma or tab: Quartz (+chert/quartzite) (—), Feldspar (—), Lithic (Rock) Fragments (—)
⚠ Needs at least 1 valid row to calculate — 0 so far.
Description
Classifies one or more point-counted sandstone (arenite) samples by their relative quartz, feldspar, and lithic-fragment framework-grain composition, against Folk (1968)'s standard 7-field QFL classification (Quartzarenite, Subarkose, Sublitharenite, Arkose, Lithic Arkose, Litharenite, Feldspathic Litharenite). Each pasted row is normalized to its own Q+F+L=100% before classification, so raw point-count totals and pre-computed percentages both work. This session ships the classification mechanism only, with no chart rendering yet — the ternary-triangle plot itself is a separately-scoped follow-up (see ARCHITECTURE.md's QFL Ternary Diagram precheck).
Variables
| Symbol | Unit | Description |
|---|---|---|
| Class | — | Sample Classifications (Folk, 1968) |
| Q, F, L | — | One row per point-counted thin section or sample: relative quartz, feldspar, and lithic-fragment abundance. Either raw grain-count totals or pre-computed percentages are accepted — each row is auto-normalized to its own Q+F+L=100% before classification, so the two input styles produce identical results. A row with a non-positive or negative sum is flagged and skipped, not silently coerced. |
Assumptions
- Each row represents a genuine point-count (or equivalent modal analysis) of framework grains only — matrix, cement, and non-framework constituents are already excluded from Q/F/L, consistent with Folk's own definition
- Q includes chert and quartzite fragments alongside monocrystalline/polycrystalline quartz, and F includes both potassium feldspar and plagioclase, per Folk's original framework-grain grouping
- A row's Q+F+L need not already sum to 100 — auto-normalization assumes the three pasted values are mutually consistent counts/percentages of the same sample, not independently-scaled numbers
Limitations
- Classifies composition only — texture, sorting, and cementation (which distinguish an arenite from a wacke, Folk's <15% matrix cutoff) are not evaluated; this calc assumes an arenite-grade sample was pasted
- No visual/ternary-plot rendering yet — this session ships only the underlying classification table, not the triangle diagram itself
- A sample straddling a boundary within point-counting error (typically a few percent) may classify differently from a replicate count of the same thin section — treat a near-boundary result as provisional
Use Cases
- → Sandstone provenance/reservoir-quality screening: Classify point-count data from thin sections to characterize reservoir sandstone composition — e.g. distinguishing a clean quartzarenite (typically better-sorted, higher-quality reservoir rock) from a feldspar- or lithic-rich sandstone more prone to diagenetic clay/matrix degradation of porosity.
- → Multi-sample batch classification: Paste an entire well's or field's worth of point-count results at once (up to 50 samples) and get a per-sample classification table, rather than manually plotting each point on a printed ternary chart.
Related Calculations
Region Notes
Global
Folk's QFL classification is compositional naming only — it says nothing about tectonic provenance. For provenance interpretation (craton interior, recycled orogen, magmatic arc, etc.), a separate Dickinson & Suczek (1979)/Dickinson (1985)-style provenance diagram is needed, which uses different, less precisely-defined field boundaries and is not implemented by this calc.
References
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