Identifying Sandstone Type from Modal Analysis

The modal analysis provides the composition of the sandstone:

Sandstone Classification Rules

Sandstone classification primarily depends on the relative proportions of framework grains (Quartz, Feldspar, Rock fragments) and the matrix content.

• Matrix Content: Sandstones with less than 10-15% matrix are classified as arenites. Those with greater than 10-15% matrix are classified as wackes.
• Framework Grain Composition (for Arenites):
  • Arkose: Contains at least 25% feldspar.
  • Sub-arkose: Contains 10% to 25% feldspar.
  • Quartz Arenite: Contains less than 10% feldspar and rock fragments combined (often >90% quartz).

Applying Classification Criteria

Based on the given modal analysis:

• The matrix content is 8%. Since 8% is less than the typical 10-15% threshold, this sandstone is classified as an arenite.
• The framework grains are Quartz (36%) and Feldspar (55%).
• The feldspar content is 55%.

Since the feldspar content (55%) is significantly greater than the 25% threshold for arkose, and the matrix content is low (8%), the sandstone fits the definition of an Arkose.

Calculation check for Feldspar ratio: $\frac{\text{Feldspar}}{\text{Feldspar} + \text{Quartz}} = \frac{55}{55 + 36} = \frac{55}{91} \approx 0.60$. This ratio confirms the dominance of feldspar over quartz among the major framework components.

Conclusion

The sandstone, with 55% feldspar and 8% matrix, is classified as Arkose.

Correct Option: A