OPTICAL

The optical tool can perform custom data comparison on the various properties observed when a mineral is exposed to polarized light. All the properties discussed require advanced knowledge in mineral optics and are obtained via specialized optical equipment. This tool is more applicable in a lab setting where you can obtain the search parameters required. The properties supported are Optic classes, Optic signs, Refractive indices, 2V Angles, Dispersion and Birefringence. If you know any one of these properties, the tool will be able to perform a database search versus known mineral optical data. Tapping the always persistent blue floating button on the lower right will immediately start a database search based on your checked options.

Optic class

The optic class illustrates on how the refractive index relates to crystal structure, manifested as Isotropic, Uniaxial or Biaxial classes. If you know your mineral’s optic class, you can use this tool to make a database comparison with known optic classes. In most scenarios, the optic class alone is insufficient to make a positive identification of a mineral and requires additional parameters such as birefringence. In order to match for optic classes, start by…

• Tapping the 'Optic Class' checkbox to activate the 'Choose class' dropdown menu on its right.

• Tap the dropdown menu to see your available options.

• Tap the class of your choice to confirm your selection.

Should you choose the Isotropic class; all other options apart from refractive indices will become disabled since isotropic minerals do not support them. Selecting the Uniaxial class will disable the 'Dispersion' and '2V angles', since they are also not applicable to that class.

Optic Sign

Optic signs are exclusive to anisotropic minerals which can be positive, negative or neutral. This property is disabled should you choose the isotropic option on the 'Optic class' property above. To match an optic sign, do…

• Make sure the Isotropic option under 'Optic Class' is not selected.

• Tap the 'Optic Sign' checkbox to activate the 'Choose sign' dropdown menu on its right.

• Tap the dropdown menu to see the available optic signs.

• Tap the sign of your choice to confirm your selection.

2V Angles

2V angles are the differences (in degrees) between the biaxial optical axes of a mineral. These angles can be measured (under microscope) or calculated. As per definition, these angles are exclusive to biaxial minerals and will be disabled if any other class apart from 'Biaxial' is selected under 'Optic Class' above. To match 2V angles, do…

• Tap any of the 2V Angle(..) checkboxes to activate their corresponding text fields on their right.

• Input the 2V angle on the now available text input area.

• Since all 2V angles in the database are Acute Bisectrix Angles (BXA), the text input filed will not allow angles greater than 90. It will further limit your inputs to digits greater than or equal to 0 only.

Some minerals have a range of angles for both their measured and calculated 2V angles. In such situations, this tool will test if your inputted angle falls within this range.

Refractive Indices (R.I.)

A refractive index is the ratio on the speed of light in vacuum C, to the speed of light in a substance through which it passes Cmatter. For anisotropic minerals, this ratio can manifest its self between two extremes depending on the optic class. The 'R.index' checkbox allows you to do a database comparison with known mineral refractive indices. This option can be further customized by selecting your target optic class from the 'Optic class' option above. That is, by not choosing an optic class, the tool will simply attempt to match all the refractive indices matching the provided search index. However, should you choose an optic class; the tool will respond by matching for a specific extreme index. 

Example; if you select the uniaxial optic class, then you will gain an opportunity to search for either the ε or ω index independently. Not selecting any class will match any index falling between ε and ω and any other optic class.

To perform a general search independent of optic classes, do so by…

• Tapping the 'R.index' checkbox to activate its corresponding text field below it.

• Input your index on the now available text input area.

• Since all indices in the database are less than 5, the text field will further limit your inputs to digits greater than 1 and less than 5.

• Tap the 'Search' button to get your results.

To match a specific index dependent of optic class, do so by…

• Tapping the 'Optic class' checkbox then choose your desired class from its drop down menu.

• Depending on the class selected above, you will now have 1 to 3 active checkboxes to choose from. 

• Tap your desired index checkbox to activate its corresponding text field below it.

• Input your index on the now available text input area. It is Ok to leave other options unselected.

• Tap the 'Search' button to get your results.

Some minerals’ indices may have a range of values on each of their index. In such situations, this tool will test if your provided index falls within this range and match accordingly.

Dispersion

Dispersion is the separation of a ray or beam of white light into its component colors. Under polarized light, dispersion is noted by the color fringing represented by the relative position of 2Vr for red light and 2Vv for violet light (2V in this case being the angle). Since 2V angles are required to notice dispersion, this property is again exclusive to biaxial minerals and will be disabled if any other class apart from 'Biaxial' is selected under 'Optic Class' above. If dispersion is noticed on your sample, you can compare it with known mineral dispersion by… 

• Tapping the 'Dispersion' checkbox to activate the 'Choose dispersion' dropdown menu on its right.

• Tap the dropdown menu to see the available dispersion relationships.

• Tap the red to violet relationship of your choice to confirm selection.

Birefringence

Birefringence is the maximum difference between refractive indices seen by the material hence more than one index of refraction is required to calculate it. This limits this property to anisotropic minerals only. To compare your birefringence against the database, do…

• Make sure the Isotropic option under 'Optic Class' is not selected.

• Tap the 'Birefringence' checkbox to enable the text input are a on its right.

• Input your calculated birefringence on the provided text input area.

• The text field has filters to allow only numerical values for processing limited to 0–1.5 range which is more than the highest difference you can have on mineral birefringence.

Some minerals have different ranges of their birefringence. In such situations, this tool will test if your inputted birefringence falls within this range.

Optical result card

When satisfied with your search parameters, tap the floating button at the lower right of the tool to begin the search. If results are found, you will receive a list of cards matching all your parameters. The cards will also provide a summary of the properties it has. Tapping any of the cards will direct you to the 'Mineral details' page of the mineral.