TetraScope

basalt tetrahedron (Yoder & Tilley 1962) and CMAS projections (O'Hara 1968)

Composition (wt %)

Projection

Dataset

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or paste / upload CSV with a header row (label, SiO2, TiO2, …). Quoted fields, semicolon- or tab-separated files are fine. Descriptive columns (rock, locality, total, …) are skipped; any other unknown column stops the import so that a typo such as Si02 cannot silently drop an oxide.

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    Where the composition sits

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    Projected position

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    Components and norm

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    Assumptions applied

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    Documentation

    Open the full manual — scientific background (the basalt tetrahedron, thermal divides, O'Hara's CMAS reduction, projection geometry and mathematics), how to read each diagram, the CIPW norm, classification, validation against Yoder & Tilley's Table 2 and O'Hara's own figures, and good practice.

    1. Enter a composition

    Type oxide weight per cent (or load an example / a Table 2 analysis). Give FeO and Fe2O3 separately, or total iron as FeO* — then the Fe3+/ΣFe ratio splits it. Negative, non-numeric or non-finite values are rejected; a total outside 97–102 wt% is accepted with a warning.

    2. Read where it sits

    The composition is reduced to O'Hara's C, M, A, S components and to the Di–Ol–Ne–Qz normative tetrahedron (via a CIPW norm). The right-hand panel names its Yoder & Tilley group and which side of each dividing plane it lies on.

    3. Project it

    Choose a preset diagram, or build any projection: one projection point and three target vertices gives a ternary; two projection points and two vertices gives a binary section. Choices where the projection point lies in the target plane are refused with a reason; nearly-coplanar choices are flagged as ill-conditioned.

    4. Plot datasets

    Load Yoder & Tilley's Table 2, paste or upload a CSV, or plot your saved collection. Export the diagram as SVG/PNG and the coordinates as CSV.

    Units

    Plotted coordinates are weight per cent of the target vertices, as in O'Hara's figures. The "formula units" column gives the same point in molecular units; the projection-point coefficient says how much of that phase must be added or subtracted to reach the plane.