When the Model Fits, but the Material Doesn’t - A Reformix Case Study on Hands-On Material Characterisation

A qXRD model reported up to 68 wt.% crystalline silica in an opaline deposit. The statistical fit looked acceptable. The material told a different story. X-ray diffraction (XRD) is a powerful analytical tool, but it is not self-validating.

In a recent materials characterisation project, Reformix reviewed a quantitative XRD (qXRD) analysis of an amorphous opaline silica deposit. The Rietveld refinement model indicated significant crystalline silica phases, including cristobalite and tridymite, with crystalline content reported as high as 68 wt.%.

Bulk Physicochemical Material Testing, such as via X-Ray Fluoresence (XRF) [Pictured above], is essential for understanding the bulk chemistry of the material, but it does not show the full picture and cannot always infer reactivity. Other complementary tests are needed.

On paper, the statistical fit appeared acceptable.

In practice, the interpretation did not align with the material and the misinterpreted results could've been catastrophic and costly (incorrect RCS reporting, distorted reactivity assumptions and poor product-development decisions).

The Context Problem

The geological history of the deposit indicated no high-temperature processing for the formation of crystalline polymorphs such as tridymite. SEM observations and internal qXRD from Reformix further supported a predominantly amorphous morphology and composition.

The discrepancy arose from a common analytical limitation:

Crystalline solids exhibit peak-like characteristics and have their mathematical parameters crystallographically defined for qXRD modelling. Amorphous materials are usually characterised by broad diffraction humps and are usually never mathematically fitted as crystalline phases... But for materials in the middle (termed mineraloids), that is, not fully amorphous nor completely ordered crystals; the humps become narrow and are commonly mistaken for crystalline peaks. If one is mistaken for the other, misinterpreted results are inevitable - no matter how good your statistics are.

A sample X-Ray Diffractogram obtained showing crystalline and mineraloid 'peaks' above a smaller amorphous hump.

Independent Validation

To resolve the inconsistency, Reformix requested chemical dissolution testing as a complementary test.

Under this method:

• It can be assumed that the amorphous material dissolves; and

• Cristobalite and other crystalline SiO₂ polymorphs do not.

The result showed a whopping 61 wt.% mass loss - directly confirming a substantial amorphous fraction inconsistent with the previously modelled crystalline content.

Following this validation, the XRD interpretation was revised to reflect the material’s actual phase composition.

Why This Matters

Misclassifying amorphous silica as crystalline phases has serious practical consequences, including:

• Inflated respirable crystalline silica (RCS) reporting

• Distorted reactivity assumptions

• Misguided product development decisions

• Incorrect regulatory positioning

Rietveld refinement is a modelling tool. Like any model, it requires boundary conditions informed by chemistry, geology and microscopy - not just mathematics.

250 & 140 μm view field Scanning Electron Microscopy (SEM) Comparison between amorphous silica (AS) samples and qualitative crystallites by Reformix Materials Group ©.

The Reformix Approach

At Reformix, material characterisation integrates:

• Quantitative XRD

• SEM and microstructural assessment

• Dissolution chemistry

• Geological process context

• Critical evaluation of modelling assumptions

Material Characterisation is not about producing the neatest model. It is about determining whether the model represents the material well enough to support the decision being made.