BH250-38b
Title
BH250-38b
Subject
Topaz Bxo
Description
BH250-38 and BH250-38b show the BXA and BXO optic figures for topaz, respectively. These two types of interference figures are key to understanding the optical properties of biaxial minerals like topaz.
What are BXA and BXO?
The BXA (Biaxial Axial) optic figure is observed when looking directly down one of the optic axes of a biaxial mineral. In contrast, the BXO (Biaxial Optic Normal) optic figure appears when viewing the mineral perpendicular (normal) to the optic axes, typically along the acute bisectrix (Bxa) or obtuse bisectrix (Bxo). Topaz, being a biaxial positive mineral with orthorhombic symmetry, exhibits both these characteristic optic figures under a petrographic microscope.
How to Obtain BXA and BXO Figures
To observe these figures, prepare a thin section of topaz around 30 microns thick, ideally oriented to reveal interference figures.
To locate the BXA figure, identify a grain oriented so that you are looking directly down the optic axis—usually along the acute or obtuse bisectrix directions (Bxa or Bxo). Center the figure by rotating the microscope stage.
To find the BXO figure, adjust the stage to view the mineral perpendicular to the optic axis, i.e., along the optic normal. This orientation is often aligned with the crystallographic axis b or along the bisectrix directions. The isogyres you observe in this position form a distinct pattern different from the BXA figure.
Optical Properties in BXA and BXO Figures
The BXA figure displays two curved isogyres (black crosses) that approach each other near the center but do not merge. The center of the figure marks the melatope, the point where the optic axis emerges. These isogyres move noticeably when the stage is rotated, showing typical biaxial mineral behavior.
The BXO figure, by comparison, shows isogyres forming a centered cross that splits and merges upon stage rotation. The isogyres usually intersect at right angles and can appear as straight or curved lines depending on orientation and birefringence. The BXO figure is generally simpler in appearance than the BXA figure.
Steps to Determine Which Figure You Are Observing
Start by identifying the grain orientation: a BXA figure is seen when looking down the optic axis (along the bisectrix), while a BXO figure appears when looking perpendicular to the optic axis (optic normal).
Next, observe the behavior of the isogyres during stage rotation. In BXA figures, the isogyres curve and approach but never cross at the center, creating a characteristic figure-eight shape. In BXO figures, isogyres form a cross that splits and merges more straightforwardly.
Check the interference colors and the presence of melatopes: BXA figures show two melatopes (optic axes), while BXO figures do not show melatopes since the view is perpendicular to the optic axis.
Finally, using accessory plates like a sensitive tint or mica plate can help determine the optic sign and provide additional characterization of the figure.
What are BXA and BXO?
The BXA (Biaxial Axial) optic figure is observed when looking directly down one of the optic axes of a biaxial mineral. In contrast, the BXO (Biaxial Optic Normal) optic figure appears when viewing the mineral perpendicular (normal) to the optic axes, typically along the acute bisectrix (Bxa) or obtuse bisectrix (Bxo). Topaz, being a biaxial positive mineral with orthorhombic symmetry, exhibits both these characteristic optic figures under a petrographic microscope.
How to Obtain BXA and BXO Figures
To observe these figures, prepare a thin section of topaz around 30 microns thick, ideally oriented to reveal interference figures.
To locate the BXA figure, identify a grain oriented so that you are looking directly down the optic axis—usually along the acute or obtuse bisectrix directions (Bxa or Bxo). Center the figure by rotating the microscope stage.
To find the BXO figure, adjust the stage to view the mineral perpendicular to the optic axis, i.e., along the optic normal. This orientation is often aligned with the crystallographic axis b or along the bisectrix directions. The isogyres you observe in this position form a distinct pattern different from the BXA figure.
Optical Properties in BXA and BXO Figures
The BXA figure displays two curved isogyres (black crosses) that approach each other near the center but do not merge. The center of the figure marks the melatope, the point where the optic axis emerges. These isogyres move noticeably when the stage is rotated, showing typical biaxial mineral behavior.
The BXO figure, by comparison, shows isogyres forming a centered cross that splits and merges upon stage rotation. The isogyres usually intersect at right angles and can appear as straight or curved lines depending on orientation and birefringence. The BXO figure is generally simpler in appearance than the BXA figure.
Steps to Determine Which Figure You Are Observing
Start by identifying the grain orientation: a BXA figure is seen when looking down the optic axis (along the bisectrix), while a BXO figure appears when looking perpendicular to the optic axis (optic normal).
Next, observe the behavior of the isogyres during stage rotation. In BXA figures, the isogyres curve and approach but never cross at the center, creating a characteristic figure-eight shape. In BXO figures, isogyres form a cross that splits and merges more straightforwardly.
Check the interference colors and the presence of melatopes: BXA figures show two melatopes (optic axes), while BXO figures do not show melatopes since the view is perpendicular to the optic axis.
Finally, using accessory plates like a sensitive tint or mica plate can help determine the optic sign and provide additional characterization of the figure.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 38b. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-38b,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/43.