L* 86–93

Whiteness Range

Standard to Premium Grade K-Spar

a* 0±0.8

Red/Green Axis

Near-neutral: no green/red cast

b* 0–3.5

Yellow/Blue Axis

Slight warm tint in standard grades

99.5%

SiO₂ in Quartz

Ultra-pure quartz L* 94–97

L*

Lightness / Whiteness

0 (black) → 100 (white)

Most critical: higher L* = whiter feldspar = brighter fired ceramic body and glaze

a*

Red (+) / Green (−) axis

−128 → +128

Near zero is ideal; positive a* = reddish tint (Fe contamination); negative = greenish

b*

Yellow (+) / Blue (−) axis

−128 → +128

Slight positive b* (0–3.5) is normal in feldspar; high b* = yellowish tint indicating iron/organic content

Key Formula: ΔE* (Total Colour Difference)

ΔE* = √[(ΔL*)² + (Δa*)² + (Δb*)²]

ΔE* < 1.0 = imperceptible difference | ΔE* 1–3 = slight difference | ΔE* > 3 = clearly visible difference

Ceramic manufacturers typically specify ΔE* < 1.5 between batches for consistent production.

L* (Whiteness)

88.5

86.0 – 90.5

Good whiteness; suitable for standard floor tile body and glaze applications

a* (Red/Green)

0.2

−0.2 → +0.5

Near-neutral; minor positive a* from trace iron; acceptable for most ceramics

b* (Yellow/Blue)

2.8

1.5 → 4.0

Slight warm yellow tone; normal for commercial-grade feldspar with Fe₂O₃ ~0.12–0.18%

Fe₂O₃ (correlated)

0.12–0.18%

0.10–0.25%

Direct relationship: higher Fe₂O₃ = lower L* and higher b*

L* (Whiteness)

91.5

90.0 – 93.0

Excellent whiteness; preferred for premium tiles, sanitaryware glaze, and bone china

a*

0.1

−0.1 → +0.3

Virtually neutral; no perceptible colour cast

b*

1.8

1.0 → 2.5

Very low yellow tint; high-clarity fired surface

Fe₂O₃ (correlated)

0.06–0.10%

0.04–0.12%

Wet magnetic separation required to achieve this grade

The Iron-Whiteness Relationship — A Critical Correlation for QC Managers

Fe₂O₃ 0.10% → L* ~91–93, b* ~1.5–2.0 | Fe₂O₃ 0.15% → L* ~88–90, b* ~2.5–3.5 | Fe₂O₃ 0.25% → L* ~84–87, b* ~4.0–6.0

Every 0.05% increase in Fe₂O₃ typically reduces L* by approximately 1.0–1.5 units and increases b* by ~0.8–1.2 units in potash feldspar.

Ajmer District

90–93

0.0 to +0.3

1.2–2.2

0.06–0.10

Premium low-iron; best for frits, bone china, sanitaryware

Bhilwara District

88–91

0.1 to +0.4

2.0–3.0

0.10–0.15

Good standard grade; widely exported to ASEAN

Sikar District

87–90

0.1 to +0.5

2.5–3.8

0.12–0.18

Commercial grade; tile body and glaze applications

Rajsamand District

91–93

-0.1 to +0.2

1.0–2.0

0.05–0.09

Ultra-premium; selective mine extraction required

Pali / Barmer

85–88

0.2 to +0.6

3.0–4.5

0.18–0.28

Standard commercial; paint and filler applications

Standard Albite

90–92

0.0 to +0.3

1.5–2.8

0.08–0.12

Jamnagar, Gujarat

Wall tile glaze, floor tile body

Premium Albite

92–94

−0.1 to +0.2

0.8–1.5

0.04–0.08

Porbandar, Gujarat

Glass frit, bone china, fine porcelain

Super Premium Albite

93–95

−0.2 to +0.1

0.5–1.0

0.02–0.05

Saurashtra select mines

Semiconductor glass, specialty ceramics

Why Soda Feldspar Achieves Higher L* Than Potash Feldspar

Albite (NaAlSi₃O₈) has a higher SiO₂ content (67–69% vs 64–66% in K-spar) and a different crystal structure that accommodates less iron substitution in the lattice. Gujarat's Saurashtra belt produces particularly clean albite with naturally low Fe₂O₃, making it the whitest naturally occurring feldspar commercially available from India.