Sample composition

Examples

Didactic examples placed in representative fields; they are neither type compositions nor reference analyses.

SiO₂Silica

wt %

Na₂OSodium oxide

wt %

K₂OPotassium oxide

wt %

Analytical basis

TAS provides a non-genetic chemical classification of volcanic rocks. Alteration and Na or K mobility may shift a sample between fields.

In the IUGS system, modal classification takes priority when the mineralogy can be determined reliably. This scheme and its names apply to volcanic rocks.

IUGS_TAS_1986_2002

Results

TAS name

Basalt

BWithin a TAS field

Normalised SiO₂

49.5

wt %

Na₂O + K₂O

3.2

wt %

Na₂O

2.6

wt %

K₂O

0.6

wt %

Normalisation factor

dimensionless

Distance to boundary

1.8

wt %

Boundary between B · S1

TAS diagram

Representation based on the IUGS TAS scheme (Le Bas et al., 1986; Le Maitre et al., 2002).

Fundamentals & Explanation

Scope of TAS classification

The TAS (total alkali–silica) diagram is a chemical classification of volcanic rocks. In the IUGS system, primary classification is based on modal mineral composition; when the mode cannot be determined reliably — for example, in glassy or very fine-grained rocks — whole-rock chemical composition may be used.

The scheme of Le Bas et al. (1986) is non-genetic: it assigns a compositional name and does not, by itself, imply a tectonic setting, magmatic series or petrogenetic process.

Horizontal axis

x = \mathrm{SiO_2}

Vertical axis

y = \mathrm{Na_2O} + \mathrm{K_2O}

Anhydrous basis and normalisation

TAS coordinates are expressed in weight percent recalculated to 100 % on an anhydrous basis. H₂O and CO₂ are excluded and the complete set of remaining oxides is normalised. If the analysis is already normalised, the three values can be used directly; otherwise a single factor is applied to SiO₂, Na₂O and K₂O:

f = \frac{100}{\sum \text{anhydrous oxides}} \qquad X_i^{norm} = X_i\,f

The denominator includes all major anhydrous oxides, not only SiO₂, Na₂O and K₂O. Normalising those three components alone does not represent a whole-rock composition and artificially changes the position on the diagram.

Fields and additional names

The scheme contains 15 fields. A point determines the root field, but some names require criteria that are not contained in the two TAS axes:

U1 — Tephrite/Basanite: separation depends on normative olivine calculated from a CIPW norm.
T — Trachyte/Trachydacite: separation depends on normative quartz q.
B — Basalt: the TAS axes alone do not distinguish alkaline, tholeiitic or calc-alkaline series.

In fields S1–S3, the Na–K criterion of the adopted edition provides sodic or potassic sub-root names:

\mathrm{Na_2O} - 2 \geq \mathrm{K_2O} \Rightarrow \text{sodic} \qquad \mathrm{Na_2O} - 2 < \mathrm{K_2O} \Rightarrow \text{potassic}

S1: hawaiite / potassic trachybasalt · S2: mugearite / shoshonite · S3: benmoreite / latite. The root field name is retained alongside the applicable sub-root name.

Boundaries and interpretation

If a composition lies on a shared edge, there is no single geometric field to which it can be assigned. All fields meeting at that boundary are therefore reported. An edge belonging to one field only is identified as the outer boundary of the TAS domain.

The reported distance is the Euclidean separation from the nearest limit in the (SiO₂, Na₂O+K₂O) plane, expressed in wt %. It is a geometric measure, not a substitute for analytical uncertainty. In altered samples, Na and K mobility may shift the plotted point, so the name should be interpreted with caution.

In brief

Data: SiO₂, Na₂O and K₂O in weight percent.
Basis: complete analysis normalised to 100 % anhydrous.
Coordinates: SiO₂ against Na₂O + K₂O.
Output: TAS field, applicable qualifier and position relative to field limits.

Applicability

TAS is used for volcanic rocks when a reliable modal composition is unavailable. If the minerals can be identified and quantified, modal classification occupies the primary level in the IUGS system. The nomenclature of this diagram does not transfer directly to plutonic rocks.

Sample quality

Hydrothermal alteration, weathering and other secondary processes may mobilise Na and K. Formal classification is best based on fresh samples and analytical data with a documented normalisation basis.

Scheme version

IUGS_TAS_1986_2002: the scheme of Le Bas et al. (1986), incorporated into the recommendations compiled by Le Maitre et al. (2002).

Sources

Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. & Zanettin, B. (1986). A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram. Journal of Petrology, 27(3), 745–750.

Le Maitre, R. W. et al. (2002). Igneous Rocks: A Classification and Glossary of Terms (2nd ed.). Cambridge University Press.

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TAS Volcanic Rock Classification

Sample composition

Results

TAS diagram

Fundamentals & Explanation

Scope of TAS classification

Anhydrous basis and normalisation

Fields and additional names

Boundaries and interpretation

In brief

Applicability

Sample quality

Scheme version

Sources

Scope of TAS classification

Anhydrous basis and normalisation

Fields and additional names

Boundaries and interpretation

In brief

Applicability

Sample quality

Scheme version

Sources