# Source code for burnman.optimize.composition_fitting

```
# This file is part of BurnMan - a thermoelastic and thermodynamic toolkit for
# the Earth and Planetary Sciences
# Copyright (C) 2012 - 2021 by the BurnMan team, released under the GNU
# GPL v2 or later.
from __future__ import absolute_import
import numpy as np
from .linear_fitting import weighted_constrained_least_squares
[docs]def fit_composition_to_solution(solution,
fitted_variables,
variable_values, variable_covariances,
variable_conversions=None,
normalize=True):
"""
Takes a Solution object and a set of variable names and
associates values and covariances and finds the molar fractions of the
solution which provide the best fit (in a least-squares sense)
to the variable values.
The fitting applies appropriate non-negativity constraints
(i.e. no species can have a negative occupancy on a site).
Parameters
----------
solution : burnman.Solution object
The solution to use in the fitting procedure.
fitted_variables : list of strings
A list of the variables used to find the best-fit molar fractions of
the solution. These should either be elements such as "Fe",
site_species such as "Fef_B" which would correspond to a
species labelled Fef on the second site,
or user-defined variables which are arithmetic sums of
elements and/or site_species defined in "variable_conversions".
variable_values : numpy array
Numerical values of the fitted variables.
These should be given as amounts; they do not need to be normalized.
variable_covariances : 2D numpy array
Covariance matrix of the variables.
variable_conversions : dictionary of dictionaries or None
A dictionary converting any user-defined variables into an
arithmetic sum of element and site-species amounts. For example,
{'Mg_equal': {'Mg_A': 1., 'Mg_B': -1.}}, coupled with Mg_equal = 0
would impose a constraint that the amount of Mg would be equal on
the first and second site in the solution.
normalize : boolean (default: True)
If True, normalizes the optimized molar fractions to sum to unity.
Returns
-------
popt : numpy array
Optimized molar fractions.
pcov : 2D numpy array
Covariance matrix corresponding to the optimized molar fractions.
res : float
The weighted residual of the fitting procedure.
"""
n_vars = len(fitted_variables)
n_mbrs = len(solution.endmembers)
solution_variables = solution.elements
solution_variables.extend(solution.solution_model.site_names)
solution_matrix = np.hstack((solution.stoichiometric_matrix,
solution.solution_model.endmember_noccupancies))
n_sol_vars = solution_matrix.shape[1]
if variable_conversions is not None:
solution_matrix = np.hstack((solution_matrix,
np.zeros((solution_matrix.shape[0],
len(variable_conversions)))))
for i, (new_var, conversion_dict) in enumerate(variable_conversions.items()):
assert (new_var not in solution_variables)
solution_variables.append(new_var)
for var in conversion_dict.keys():
solution_matrix[:, n_sol_vars+i] += solution_matrix[:, solution_variables.index(var)]
# Now, construct A using the fitted variables
A = np.zeros((n_vars, solution_matrix.shape[0]))
for i, var in enumerate(fitted_variables):
A[i, :] = solution_matrix[:, solution_variables.index(var)]
b = variable_values
Cov_b = variable_covariances
# Define the constraints
# Ensure that element abundances / site occupancies
# are exactly equal to zero if the user specifies that
# they are equal to zero.
S, S_index = np.unique(A, axis=0, return_index=True)
S = np.array([s for i, s in enumerate(S)
if np.abs(b[S_index[i]]) < 1.e-10
and any(np.abs(s) > 1.e-10)])
equality_constraints = [S, np.zeros(len(S))]
# Ensure all site occupancies are non-negative
T = np.array([-t for t in np.unique(solution.solution_model.endmember_occupancies.T, axis=0)
if any(np.abs(t) > 1.e-10)])
inequality_constraints = [T, np.zeros(len(T))]
popt, pcov, res = weighted_constrained_least_squares(A, b, Cov_b,
equality_constraints,
inequality_constraints)
if normalize:
sump = sum(popt)
popt /= sump
pcov /= sump * sump
res /= sump
# Convert the variance-covariance matrix from endmember amounts to
# endmember proportions
dpdx = (np.eye(n_mbrs) - popt).T # = (1. - p[i] if i == j else -p[i])
pcov = dpdx.dot(pcov).dot(dpdx.T)
return (popt, pcov, res)
[docs]def fit_phase_proportions_to_bulk_composition(phase_compositions,
bulk_composition):
"""
Performs weighted constrained least squares on a set of phase compositions
to find the amount of those phases that best-fits a given bulk composition.
The fitting applies appropriate non-negativity constraints
(i.e. no phase can have a negative abundance in the bulk).
Parameters
----------
phase_compositions : 2D numpy array
The composition of each phase. Can be in weight or mole amounts.
bulk_composition : numpy array
The bulk composition of the composite.
Must be in the same units as the phase compositions.
Returns
-------
popt : numpy array
Optimized phase amounts.
pcov : 2D numpy array
Covariance matrix corresponding to the optimized phase amounts.
res : float
The weighted residual of the fitting procedure.
"""
n_phases = len(phase_compositions[0])
inequality_constraints = [-np.eye(n_phases), np.zeros(n_phases)]
popt, pcov, res = weighted_constrained_least_squares(phase_compositions,
bulk_composition,
None,
None,
inequality_constraints)
return (popt, pcov, res)
```